[ty] Improve invalid-type-form diagnostic where a module-literal type is used in a type expression and the module has a member which would be valid in a type expression (#18244)

crates/ty_python_semantic/resources/mdtest/annotations/invalid.md

@@ -113,3 +113,35 @@ def _(
     reveal_type(f)  # revealed: Unknown
     reveal_type(g)  # revealed: Unknown
 ```
+
+## Diagnostics for common errors
+
+<!-- snapshot-diagnostics -->
+
+### Module-literal used when you meant to use a class from that module
+
+It's pretty common in Python to accidentally use a module-literal type in a type expression when you
+*meant* to use a class by the same name that comes from that module. We emit a nice subdiagnostic
+for this case:
+
+`foo.py`:
+
+```py
+import datetime
+
+def f(x: datetime): ...  # error: [invalid-type-form]
+```
+
+`PIL/Image.py`:
+
+```py
+class Image: ...
+```
+
+`bar.py`:
+
+```py
+from PIL import Image
+
+def g(x: Image): ...  # error: [invalid-type-form]
+```

crates/ty_python_semantic/resources/mdtest/snapshots/invalid.md_-_Tests_for_invalid_ty…_-_Diagnostics_for_comm…_-_Module-literal_used_…_(652fec4fd4a6c63a).snap

@@ -0,0 +1,62 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: invalid.md - Tests for invalid types in type expressions - Diagnostics for common errors - Module-literal used when you meant to use a class from that module
+mdtest path: crates/ty_python_semantic/resources/mdtest/annotations/invalid.md
+---
+
+# Python source files
+
+## foo.py
+
+```
+1 | import datetime
+2 | 
+3 | def f(x: datetime): ...  # error: [invalid-type-form]
+```
+
+## PIL/Image.py
+
+```
+1 | class Image: ...
+```
+
+## bar.py
+
+```
+1 | from PIL import Image
+2 | 
+3 | def g(x: Image): ...  # error: [invalid-type-form]
+```
+
+# Diagnostics
+
+```
+error[invalid-type-form]: Variable of type `<module 'datetime'>` is not allowed in a type expression
+ --> src/foo.py:3:10
+  |
+1 | import datetime
+2 |
+3 | def f(x: datetime): ...  # error: [invalid-type-form]
+  |          ^^^^^^^^
+  |
+info: Did you mean to use the module's member `datetime.datetime` instead?
+info: rule `invalid-type-form` is enabled by default
+
+```
+
+```
+error[invalid-type-form]: Variable of type `<module 'PIL.Image'>` is not allowed in a type expression
+ --> src/bar.py:3:10
+  |
+1 | from PIL import Image
+2 |
+3 | def g(x: Image): ...  # error: [invalid-type-form]
+  |          ^^^^^
+  |
+info: Did you mean to use the module's member `Image.Image` instead?
+info: rule `invalid-type-form` is enabled by default
+
+```