InstalledPackageResolver: Note when a package provides no import names

Add a debug-level warning when neither _top_level_declared() nor
_top_level_inferred() is unable to find any provided import names in a
package.

This _might_ be a symptom of a weird Python environment with missing
metadata (like @layus is currently experiencing with Bazel's
rules_python rule set: In the directory structure that rules_python
makes available to build steps that depend on a Python package, the
necessary metadata files (RECORD or top-level.txt) needed by
importlib_metadata in order to determine a package's import names are
not included.

However, this situation might(?) also happen with any package that
provides executable tools rather than importable modules (think e.g.
uv, ruff, or similar).

Hence we leave this as a debug-level message for now: We don't want a
scary warning for something that might be legitimate, but a debug-level
message can still be found by a user that is e.g. debugging a case where
a declared dependency is reported as both undeclared and unused.

Author: jherland

fawltydeps/packages.py

@@ -260,6 +260,14 @@ def _from_one_env(
                 _top_level_declared(dist)  # type: ignore[no-untyped-call]
                 or _top_level_inferred(dist)  # type: ignore[no-untyped-call]
             )
+            if not imports:
+                # We have found an installed package that provides zero import
+                # names. This might be a legitimate tool/application that is
+                # installed into the Python environment without providing any
+                # importable modules, but it might also be a symptom of broken/
+                # incomplete package metadata causing importlib_metadata to not
+                # find any provided import names.
+                logger.debug("  This module does not provide any import names!")
             yield {dist.name: imports}, str(parent_dir)
 
     @cached_property