Fix import module when load plugins from file to allow some pickle cases

[acjh]

Description

1. Fix the name of the module to be imported in load_plugins_from_file.
2. Fix the missing step in import_module that inserts the module into sys.modules as shown in the recipe https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly.

Motivation and Context

Pickling a plugin that has a reference to a function or class defined within the plugin module fails.
See https://stackoverflow.com/questions/74841508/cant-pickle-class-import-of-module-failed.

How Has This Been Tested?

Added test_pickle_dumps in test_loaders.py, which fails before this fix and passes after this fix.

Limitations:

• Pickling only works for the reference if load_plugins from relative path.
• on_match is incompatible with pickle, but works with cloudpickle.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] I have read the CONTRIBUTING document.
• [x] All new and existing tests passed.

[michaelkryukov]

In relation to other issues that currently is being solved (#74), I don't think this this PR will land into master. And generally pickling objects that are not supposed to be pickled - is not a good idea. Plugin objects should never be serialized. Why is that needed?

[acjh]

I can't read #74 and it doesn't make sense when Google-translated.

For why, see the linked Stack Overflow question. Note that it's not my question and I don't use Kutana, so I can't strongly justify the use case.

Regardless of whether pickling is supported, this fixes the incorrect and incomplete import helpers.

[michaelkryukov]

Sorry for mixed languages. I'm in process of rewriting loaders.py, where importing will be implemented properly (so that classes inside loaded modules will be available to pickle). That's what what basically means #74