More refactoring for plugin manager.

[jezdez]

Description

Following https://github.com/conda/conda/pull/13057/files#r1328506001 we noticed that #13057 removed the duplication check and makes the plugin loading prone to import time side effects.

Checklist - did you ...

• [ ] Add a file to the news directory (using the template) for the next release's release notes?
• [ ] Add / update necessary tests?
• [ ] Add / update outdated documentation?

[kenodegard]

After reviewing pluggy.PluginManager.register again the only instances where a ValueError is raised is when the plugin has already been registered. Let's just silently consume this instead of converting it into a PluginError.

[jezdez]

Doc build error:

Running Sphinx v4.2.0

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/envs/13118/lib/python3.10/site-packages/sphinx/config.py", line 328, in eval_config_file
    exec(code, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/checkouts/13118/docs/source/conf.py", line 14, in <module>
    import conda
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/checkouts/13118/conda/__init__.py", line 9, in <module>
    from .__version__ import __version__
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/checkouts/13118/conda/__version__.py", line 4
    __version__ : str = version : str = '23.7.5.dev89'
                                ^
SyntaxError: invalid syntax

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/envs/13118/lib/python3.10/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/envs/13118/lib/python3.10/site-packages/sphinx/application.py", line 216, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/envs/13118/lib/python3.10/site-packages/sphinx/config.py", line 172, in read
    namespace = eval_config_file(filename, tags)
  File "/home/docs/checkouts/readthedocs.org/user_builds/continuumio-conda/envs/13118/lib/python3.10/site-packages/sphinx/config.py", line 331, in eval_config_file
    raise ConfigError(msg % err) from err
sphinx.errors.ConfigError: There is a syntax error in your configuration file: invalid syntax (__version__.py, line 4)


Configuration error:
There is a syntax error in your configuration file: invalid syntax (__version__.py, line 4)