modulegraph: Use loaders to get module content.

modulegraph's heritage includes code still based on Python 2.x capabilities on
how to find a module and get its source or code. It also contained a anomaly
regarding packages and their ``__init__``-file: If a package was detected,
it's ``__init__`` file was loaded as a module. This, while being ugly, worked
in most cases, but failed if the ``__init__`` module is an extension
module (see #5131, #4346), ending in an infinite loop. This was caused by
modulegraph distinguishing between the package and its ``__init__`` module.

The solution is to switch to "modern" loaders, both being a loader for a
specific type of modules (source, extension, etc.) and having a package
characteristic (property ``is_package()``)

This commit does the following

- In ``_find_module_path()`` no longer return "metadata" but a loader. This
  also removed huge part of this function, making it much easier to understand.
  As a side effect, this asymmetric closing of a file in a completely other
  part of the code (``_safe_import_module``) could be removed.

- Change ``_load_module`` to use the loaders.

- Merge ``_load_package`` into `__load_module``, getting rid of the anomaly
  described above.

- Adjust the test-cases to the new behavior (esp. loader instead of
  metadata-tuple and filehandle)

Please note: Since we plan to change to modulegraph2 soon anyway, I did not
spend too much time on creating a clean solution.

See #4406, closes #5131, #4346.