Implement relative imports, clean up Manifest.in.

[tbenthompson]

Closes #56, #57, #64.

[tbenthompson]

Also closes #73 and #74.

[codecov-commenter]

Codecov Report

Merging #75 (3fa4241) into main (3dbc628) will decrease coverage by 7.84%.
The diff coverage is 59.86%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   92.30%   84.46%   -7.85%     
==========================================
  Files          10       12       +2     
  Lines         429      560     +131     
==========================================
+ Hits          396      473      +77     
- Misses         33       87      +54     

Impacted Files	Coverage Δ
cppimport/__main__.py	0.00% <0.00%> (ø)
setup.py	0.00% <ø> (ø)
cppimport/checksum.py	79.62% <25.00%> (-4.69%)	⬇️
cppimport/__init__.py	78.00% <62.50%> (-12.33%)	⬇️
cppimport/templating.py	92.50% <62.50%> (-7.50%)	⬇️
cppimport/find.py	92.85% <66.66%> (+2.29%)	⬆️
cppimport/importer.py	81.17% <68.75%> (-9.74%)	⬇️
tests/test_cppimport.py	98.55% <97.43%> (-0.47%)	⬇️
cppimport/import_hook.py	100.00% <100.00%> (+10.52%)	⬆️
tests/apackage/rel_import_tester.py	100.00% <100.00%> (ø)
... and 4 more

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