fix hatchling source path detection

[alexrashed]

Motivation

plux has just recently built out an integration with hatchling.
When using plux with localstack-core, I was facing some issues with plux failing on a KeyError when trying to detect the package path:

. .venv/bin/activate; python -m plux entrypoints
entry point build mode: manual
discovering plugins and writing to .../plux.ini ...
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File ".../.venv/lib/python3.13/site-packages/plux/__main__.py", line 13, in <module>
    main()
    ~~~~^^
  File ".../.venv/lib/python3.13/site-packages/plux/__main__.py", line 9, in main
    cli.main(argv)
    ~~~~~~~~^^^^^^
  File ".../.venv/lib/python3.13/site-packages/plux/cli/cli.py", line 183, in main
    args.func(args)
    ~~~~~~~~~^^^^^^
  File ".../.venv/lib/python3.13/site-packages/plux/cli/cli.py", line 58, in entrypoints
    builder.write(fd, output_format="ini")
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/index.py", line 42, in write
    ep = discover_entry_points(self.plugin_finder)
  File ".../.venv/lib/python3.13/site-packages/plux/core/entrypoint.py", line 34, in discover_entry_points
    return to_entry_point_dict([spec_to_entry_point(spec) for spec in finder.find_plugins()])
                                                                      ~~~~~~~~~~~~~~~~~~~^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/discovery.py", line 175, in find_plugins
    return collector.find_plugins()
           ~~~~~~~~~~~~~~~~~~~~~~^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/discovery.py", line 226, in find_plugins
    for module in self.modules:
                  ^^^^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/discovery.py", line 183, in _load_modules
    for module_name in self._list_module_names():
                       ~~~~~~~~~~~~~~~~~~~~~~~^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/discovery.py", line 200, in _list_module_names
    for pkg in self.finder.find_packages():
               ~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/hatchling.py", line 118, in find_packages
    self.path, relative_package_path
    ^^^^^^^^^
  File ".../.venv/lib/python3.13/site-packages/plux/build/hatchling.py", line 147, in path
    if self.builder_config.sources[""]:
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: ''
make: *** [Makefile:76: entrypoints] Error 1

Changes

• path property now correctly derives the different ways how Hatchling's sources dict maps can be set.
• find_packages - changes how paths in hatchling are relative to the project root (not the source root)

[dfangl]

Maybe we can add here another check or TODO what happens if there is a value - so not just a source dir stripping, but a true rename.