Add function for reading and caching PEP 518 spec

[cottsay]

This code was originally part of the colcon-python-project package. I intend to bring that code into colcon-core with proper tests, and hide the functionality behind a feature flag until we're ready to make it the default.

This change specifically adds code to read and cache the pyproject.toml spec on package descriptors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.05%. Comparing base (fbbb8af) to head (32653b5).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #668      +/-   ##
==========================================
- Coverage   87.35%   87.05%   -0.30%     
==========================================
  Files          67       67              
  Lines        3921     3939      +18     
  Branches      775      696      -79     
==========================================
+ Hits         3425     3429       +4     
- Misses        392      404      +12     
- Partials      104      106       +2     

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