Add plugin_manager module for global singleton convenience

[tlambert03]

Inspired by some conversations with @kne42 at the hackathon, this PR adds a new module that serves as a convenience for accessing public methods on the global PluginManager.instance() singleton.

Everything is lazy, so importing the module does nothing to instantiate the singleton. I avoided fully autogenerating the module so as to retain type hinting and basic docs (though the full docstrings with parameters will remain in the actual PluginManager methods).

This would make a lot of the lines in napari/plugins/_npe2.py and elsewhere simpler. for example:

pm = npe2.PluginManager.instance()
for plugin_name, contribs in pm.iter_sample_data():
   ...

# becomes
for plugin_name, contribs in npe2.pm.iter_sample_data():
    ...

The only downside is that the pm.py module needs to be kept manually updated (but there's a test to let us know when we need to).

thoughts @kne42, @nclack?

[codecov]

Codecov Report

Merging #164 (a26e45a) into main (a483aea) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #164   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1730      1770   +40     
=========================================
+ Hits          1730      1770   +40     

Impacted Files	Coverage Δ
npe2/_plugin_manager.py	100.00% <ø> (ø)
npe2/plugin_manager.py	100.00% <100.00%> (ø)

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[nclack]

I'm not a big fan of using pm as a name - I tend to want to spell things out rather than abbreviate. What do you think about making the singleton transparent so things look like: npe2.iter_sample_data()?

[tlambert03]

I'm not married to pm. Doing it in the root namespace is definitely aesthetically appealing, but it scares me to use that namespace 😂! I guess I could live with from npe2 import plugin_manager as pm or something like that?

[nclack]

root namespace scares me too 👻 plugin_manager looks great.

[tlambert03]

Will merge this tommorrow with new module name. Unless @nclack you think we shouldn’t have it at all? (You never commented on that)

[nclack]

I do like the idea of the global plugin_manager instance.

[nclack]

love all the improved docstrings!