Fix entry point discovery on Python < 3.10

[cottsay]

It seems that the behavior of the importlib.metadata.entry_points function changed in Python 3.10 to automatically de-duplicate distributions, but prior to that the "shadowed" distributions were also enumerated. This change specifically ignores "shadowed" distributions so that they aren't identified as extension point overwrites.

Fixes #562
Closes #596
When released, will eventually close ros2/ci#737.

[cottsay]

Looks like a change in setuptools broke the unit tests (which is distinctly unsurprising). I'll follow up with a fix tomorrow.

[clalancette]

These are all just questions, since I'm not really familiar with all of this:

1. What does a 'distribution' mean in this context?
2. What order are 'distribution's returned from distribution()? Alphabetical? Path-length? Something else?

[cottsay]

What does a 'distribution' mean in this context?

In short: there is a 1-to-1 mapping of distributions and things that you can "pip install". Distributions can carry more than one top-level module, but you can think of distributions as a "python package" to some extent.

What order are 'distribution's returned from distribution()? Alphabetical? Path-length? Something else?

From what I can tell, the path discovery order is respected but I'm not sure the order within each path is consistent. If you have a distribution "A-1.0" on your system and it is shadowed by a distribution "A-1.1" by something on your PYTHONPATH, you should always discover "A-1.1" first.

Later versions of importlib.metadata make this discovery process easier but I think we need to drop Python 3.6 and 3.7 support to access those new functions.

[cottsay]

Unrelated fix for CI: #606

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d29f38d) 83.32% compared to head (f8439ff) 83.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
- Coverage   83.32%   83.30%   -0.02%     
==========================================
  Files          65       65              
  Lines        3760     3762       +2     
  Branches      725      727       +2     
==========================================
+ Hits         3133     3134       +1     
- Misses        551      552       +1     
  Partials       76       76              

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