[ty] Check typeshed VERSIONS for parent modules when reporting failed stdlib imports (#20908)

This is a drive-by improvement that I stumbled backwards into while
looking into

* https://github.com/astral-sh/ty/issues/296

I was writing some simple tests for "thing not in old version of stdlib"
diagnostics and checked what was added in 3.14, and saw
`compression.zstd` and to my surprise discovered that `import
compression.zstd` and `from compression import zstd` had completely
different quality diagnostics.

This is because `compression` and `compression.zstd` were *both*
introduced in 3.14, and so per VERSIONS policy only an entry for
`compression` was added, and so we don't actually have any definite info
on `compression.zstd` and give up on producing a diagnostic. However the
`from compression import zstd` form fails on looking up `compression`
and we *do* have an exact match for that, so it gets a better
diagnostic!

(aside: I have now learned about the VERSIONS format and I *really* wish
they would just enumerate all the submodules but, oh well!)

The fix is, when handling an import failure, if we fail to find an exact
match *we requery with the parent module*. In cases like
`compression.zstd` this lets us at least identify that, hey, not even
`compression` exists, and luckily that fixes the whole issue. In cases
where the parent module and submodule were introduced at different times
then we may discover that the parent module is in-range and that's fine,
we don't produce the richer stdlib diagnostic.

crates/ty_python_semantic/resources/mdtest/import/basic.md

@@ -192,6 +192,26 @@ from string.templatelib import Template  # error: [unresolved-import]
 from importlib.resources import abc  # error: [unresolved-import]
 ```
 
+## Attempting to import a stdlib submodule when both parts haven't yet been added
+
+`compression` and `compression.zstd` were both added in 3.14 so there is a typeshed `VERSIONS` entry
+for `compression` but not `compression.zstd`. We can't be confident `compression.zstd` exists but we
+do know `compression` does and can still give good diagnostics about it.
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+import compression.zstd  # error: [unresolved-import]
+from compression import zstd  # error: [unresolved-import]
+import compression.fakebutwhocansay  # error: [unresolved-import]
+from compression import fakebutwhocansay  # error: [unresolved-import]
+```
+
 ## Attempting to import a stdlib module that was previously removed
 
 <!-- snapshot-diagnostics -->

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Attempting_to_import…_(dba22bd97137ee38).snap

@@ -0,0 +1,83 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Attempting to import a stdlib submodule when both parts haven't yet been added
+mdtest path: crates/ty_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import compression.zstd  # error: [unresolved-import]
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+```
+
+# Diagnostics
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression.zstd`
+ --> src/mdtest_snippet.py:1:8
+  |
+1 | import compression.zstd  # error: [unresolved-import]
+  |        ^^^^^^^^^^^^^^^^
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression`
+ --> src/mdtest_snippet.py:2:6
+  |
+1 | import compression.zstd  # error: [unresolved-import]
+2 | from compression import zstd  # error: [unresolved-import]
+  |      ^^^^^^^^^^^
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression.fakebutwhocansay`
+ --> src/mdtest_snippet.py:3:8
+  |
+1 | import compression.zstd  # error: [unresolved-import]
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression`
+ --> src/mdtest_snippet.py:4:6
+  |
+2 | from compression import zstd  # error: [unresolved-import]
+3 | import compression.fakebutwhocansay  # error: [unresolved-import]
+4 | from compression import fakebutwhocansay  # error: [unresolved-import]
+  |      ^^^^^^^^^^^
+  |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```