[WIP] Optionally store deps.jl in a different directory

[jlapeyre]

This works together with CachePath.jl as described here to allow using different libpythons without rebuilding.

If this looks ok, I'll add documentation and an example, etc.

[codecov-commenter]

Codecov Report

Merging #991 (14aab39) into master (c77c20a) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #991      +/-   ##
==========================================
+ Coverage   68.33%   68.34%   +0.01%     
==========================================
  Files          20       20              
  Lines        2018     2025       +7     
==========================================
+ Hits         1379     1384       +5     
- Misses        639      641       +2     

Flag	Coverage Δ
unittests	68.34% <66.66%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/startup.jl	52.94% <66.66%> (-0.19%)	⬇️
src/pyinit.jl	83.00% <0.00%> (-0.68%)	⬇️
src/numpy.jl	75.30% <0.00%> (-0.34%)	⬇️
src/conversions.jl	63.56% <0.00%> (+0.24%)	⬆️

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[mkitti]

There is a test failure on system "Python / Test Julia nightly Python 3.x macos-latest x64".

Also it seems to me that the environment variables would only be checked during build or precompilation, rather than package module initialization. Is that correct?

Perhaps it might be best to use a Preferences.jl mechanism to configure how PyCall finds libpython, and perhaps one of these modes could involve looking at environment variable.

[stevengj]

Yes, I think the way to go is ideally to use Preferences.jl here.

[MilesCranmer]

Is there an existing PR for using Preferences.jl?

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Nevermind: #945