When walking the ast of a module, look for AnnAssign nodes in addition
to Assign to support assignments with type annotations, for example.
Since we have to read different attributes, split the generators into
a for loop. Existing ast.Assign nodes follow the same processing as
before.

If the directory follows a src-layout-ish, try harder to
make `package_dir` in the form `{"": "src"}`.

This might be later important for PEP 660 (e.g. when composing pth
files or symlinking the toplevel packages).

Although this situation is different from the one described in #3000,
that issue served as inspiration behind this change.

Closes #3176