Since #5 Python calls resolve only through import edges, but the
lexical importer could not produce those edges for the common forms:
- `from .util import helper` never matched (the regex needed a leading
letter), so the call went unresolved;
- `from pkg import f` looked up the stem "pkg" instead of
pkg/__init__.py;
- `from x.common import f` failed when y/common.py also existed, and
`from other.common import f` (a module the repository does not have)
resolved to y/common.py by basename.
Imports now resolve Python-specifically: leading dots are relative to
the importing package, and a dotted name maps to `a/b.py` or
`a/b/__init__.py` from the repository root or `src/`. Only a
single-component name (`import foo`) may still fall back to a unique
module basename; a dotted name never does. `from . import x` stays
unresolved (the module is in the import list, not the name).
Tests (both failed before): relative, package and duplicate-stem imports
resolve their calls to pkg/util.py, pkg/__init__.py and x/common.py;
an unknown dotted module does not bind to y/common.py.
