Handle pip package versions like pip does [rev2]

[peci1]

Continuation of #901.

This would come handy e.g. with package m2r added in ros/rosdistro#35827 , where the newest released version on pip is python3-only, but it is also offered on python2 pip on bionic.

[danielcranston]

I came across ros/rosdistro#46946 (comment) which made me remember this issue. The linked comment talks about a con with adding custom rosdep entries:

Cons: still effectively limits you to "one version" as the rosdep pip keys don't (truly) support versioning. (but if you put the like ==1.2.3 in the package name for pip it will install that version... it just breaks rosdeps ability to tell that it's already installed and skip reinstallation when you run)

My understanding is that this PR addresses this exact thing, and gives rosdep the ability to "(truly) support versioning" of pip packages.

I made some commits on top of this branch a while back, mainly to resolve merge conflicts. I've been using it with custom rosdep keys targeting explicit pip package versions, and it's working well. https://github.com/danielcranston/rosdep/tree/handle-pip-module-versions. @peci1 mind having a look at the commits? If they seem good to you feel free to adopt them here.

@emersonknapp do you have any thoughts on this?

[peci1]

Thanks, I'll have a look. Not sure about the timeframe, though.

[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.09%. Comparing base (4cb03bb) to head (c8fc9e7).
⚠️ Report is 41 commits behind head on master.

Files with missing lines	Patch %	Lines
src/rosdep2/platforms/pip.py	87.87%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
- Coverage   74.22%   72.09%   -2.14%     
==========================================
  Files          44       44              
  Lines        3376     3519     +143     
  Branches        0      696     +696     
==========================================
+ Hits         2506     2537      +31     
+ Misses        870      802      -68     
- Partials        0      180     +180     

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[peci1]

Thanks @danielcranston , I've integrated your changes.