cleanup interpreter implementation of both_libraries a bit

[dcbaker]

mostly this just changes the pic calculation to reuse the one already done in the StaticLibrary initializer, but there's also a bit of code-reuse going on here as well.

This was based on some issues I noticed in my build.py typed_kwargs series

[codecov[bot]]

Codecov Report

Merging #11150 (b39311b) into master (b249470) will decrease coverage by 0.46%. The diff coverage is n/a.

:exclamation: Current head b39311b differs from pull request most recent head f0c1f51. Consider uploading reports for the commit f0c1f51 to get more accurate results

@@            Coverage Diff             @@
##           master   #11150      +/-   ##
==========================================
- Coverage   66.07%   65.60%   -0.47%     
==========================================
  Files         414      207     -207     
  Lines       90036    45011   -45025     
  Branches    19290     9966    -9324     
==========================================
- Hits        59488    29528   -29960     
+ Misses      26003    13086   -12917     
+ Partials     4545     2397    -2148     

Impacted Files	Coverage Δ
scripts/run_tool.py	0.00% <0.00%> (-100.00%)	:arrow_down:
scripts/clangtidy.py	0.00% <0.00%> (-93.34%)	:arrow_down:
templates/cstemplates.py	35.48% <0.00%> (-64.52%)	:arrow_down:
templates/javatemplates.py	36.66% <0.00%> (-63.34%)	:arrow_down:
templates/rusttemplates.py	37.93% <0.00%> (-62.07%)	:arrow_down:
templates/dlangtemplates.py	37.93% <0.00%> (-62.07%)	:arrow_down:
templates/fortrantemplates.py	39.28% <0.00%> (-60.72%)	:arrow_down:
scripts/coverage.py	0.00% <0.00%> (-59.41%)	:arrow_down:
scripts/clangformat.py	0.00% <0.00%> (-50.00%)	:arrow_down:
modules/icestorm.py	57.14% <0.00%> (-40.00%)	:arrow_down:
... and 272 more

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[dcbaker]

@xclaesse in the interest of making this easily mergable, I've dropped everything except re-using static_library's pic calculation

[xclaesse]

LGTM

[dcbaker]

@jpakkane is this okay to go in during rc, or wait till 1.1?