[ty] Initial implementation of declaration and definition providers. (#19371)

This PR implements "go to definition" and "go to declaration"
functionality for name nodes only. Future PRs will add support for
attributes, module names in import statements, keyword argument names,
etc.

This PR:
* Registers a declaration and definition request handler for the
language server.
* Splits out the `goto_type_definition` into its own module. The `goto`
module contains functionality that is common to `goto_type_definition`,
`goto_declaration` and `goto_definition`.
* Roughs in a new module `stub_mapping` that is not yet implemented. It
will be responsible for mapping a definition in a stub file to its
corresponding definition(s) in an implementation (source) file.
* Adds a new IDE support function `definitions_for_name` that collects
all of the definitions associated with a name and resolves any imports
(recursively) to find the original definitions associated with that
name.
* Adds a new `VisibleAncestorsIter` stuct that iterates up the scope
hierarchy but skips scopes that are not visible to starting scope.

---------

Co-authored-by: UnboundVariable <unbound@gmail.com>

crates/ty_python_semantic/src/semantic_index/definition.rs

@@ -23,7 +23,7 @@ use crate::unpack::{Unpack, UnpackPosition};
 #[salsa::tracked(debug)]
 pub struct Definition<'db> {
     /// The file in which the definition occurs.
-    pub(crate) file: File,
+    pub file: File,
 
     /// The scope in which the definition occurs.
     pub(crate) file_scope: FileScopeId,