by using essentially the same logic for system site-packages, on the
assumption that system site-packages are always a subdir of the stdlib
we were looking for.
This implements mapping of definitions in stubs to definitions in the
"real" implementation using the approach described in
https://github.com/astral-sh/ty/issues/788#issuecomment-3097000287
I've tested this with goto-definition in vscode with code that uses
`colorama` and `types-colorama`.
Notably this implementation does not add support for stub-mapping stdlib
modules, which can be done as an essentially orthogonal followup in the
implementation of `resolve_real_module`.
Part of https://github.com/astral-sh/ty/issues/788
