Fix import resolution for monorepos

[emdoyle]

Fixes: #564

This PR significantly changes the semantics of check-external, and brings a much more explicit understanding of packages to Tach.

Before this PR:

• anything in any of the source_roots was defined as a 'project import', or in other words an internal import, and would not be covered by check-external
• only pyproject.toml was recognized as a package marker, and its parsed contents defined the actual source roots which would be checked (leading to strange behavior when it differed from Tach's source_roots)
• error messages about unused dependencies did not point out which package contained the issue

After this PR:

• source_roots are separate from 'packages', and will 'roll up' to the nearest package above them
• packages are identified by the presence of pyproject.toml, requirements.txt, setup.py or the Tach project root [NOTE: setup.py is not supported, since it is not always possible to check statically]
• in addition to 3rd party packages, check-external will check all imports which resolve to source code outside the current package - this covers local packages in a monorepo
• error messages about unused dependencies show the package root which declares the unused dependencies

This PR also introduces additional examples and tests. One directory uses namespace packages, while the other is drawn from the docs on uv workspaces. In both cases, imports can be resolved to the other local packages, and checked with check-external.

Upcoming work:

• glob patterns and exclusions in source_roots
• external dependency controls for modules (e.g. depends_on equivalents for external dependencies)
• dependency banning for modules
• dependency groups
• moving check-external into a flag for check