MYPYPATH missing entries for pip deps behind local alias targets

Pip packages that ship inline types (`py.typed` + `.pyi`) aren't discoverable by mypy when they're accessed through local `py_library` wrappers (the standard `rules_python` pattern for pip aliases).

The type stubs **are** in the sandbox (confirmed via `aquery`), but their directory never makes it onto `MYPYPATH`.

### Why

MYPYPATH construction in `_mypy_impl` decides how to handle a dep based on its label. There are two relevant branches:

- **External deps** (`workspace_root.startswith("external/")`) — correctly adds `external//site-packages` to the path.
- **Local deps** (`workspace_name == ""`) — assumes imports are first-party, strips the first path segment (expecting `_main/...`), and adds the remainder.

A pip alias like `//requirements/pip:whenever` looks local to Bazel (its label lives in the main repo), so it takes the second branch. But its `PyInfo.imports` are inherited from the actual external pip package and look like `rules_python++pip+pip_312_whenever_.../site-packages`. Stripping the first segment turns this into just `site-packages` — a meaningless path. The full path (`external/rules_python++pip+.../site-packages`) is never added.

==> The dep is local, but its imports are external.

Start in the _mypy_impl MYPYPATH construction described in the issue, comparing the external-dependency and local-dependency branches. Use aquery to inspect the local pip alias's PyInfo.imports and verify the sandbox path; done means mypy can discover the package's inline types through that alias.

Written by the indexing model from the issue text.