Add support for pytest fixtures from local pytest plugins.

[moser]

I like to structure my pytest fixtures in files separate from conftest.py. The pytest docs advise to include those modules containing fixtures as plugin modules, by giving the fully qualified names as strings in an array assinged to pytest_plugins in the conftest.py.

In this PR, I propose adding the modules from the above-mentioned list to the fixture source modules for better completion and type inference.

I guess that my approach in _load_pytest_plugins is a bit crude and could use some feedback and error handling. Can you give me feedback on this?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (11280ef) 94.45% compared to head (c4815c9) 94.46%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1907   +/-   ##
=======================================
  Coverage   94.45%   94.46%           
=======================================
  Files          80       80           
  Lines       11917    11932   +15     
=======================================
+ Hits        11256    11271   +15     
  Misses        661      661           

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[davidhalter]

Hmm good point. I feel like generally you could just infer instead of goto and if a list is inferred infer the inner list elements. That's probably a bit more "solid".

So the order is infer(), probably something like py__iter__?, infer()? each element and then probably check with get_str_or_none(value) if a value is a valid string that you could use.

[moser]

@davidhalter
I tried to improve on this. Can you have another look?

[davidhalter]

@moser Thanks! This looks generally good. It's still marked as a draft. So you might want to finish whatever you want to do and then I'll merge.

[moser]

@davidhalter Ready from my side :-)

[davidhalter]

Thanks a lot Martin, good stuff!