Analyze submodules (part 1)

[matsoftware]

This is the ground-up work to perform sub-modules analysis (#21).

The SwiftFile instances are now stored by keeping the path hierarchy and the framework'sSyntheticData is computed out of the parsed files.

[codecov]

Codecov Report

Merging #25 into master will increase coverage by 0.10%.
The diff coverage is 98.71%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   98.04%   98.14%   +0.10%     
==========================================
  Files           7        7              
  Lines         563      594      +31     
==========================================
+ Hits          552      583      +31     
  Misses         11       11              

Impacted Files	Coverage Δ
swift_code_metrics/_report.py	98.48% <80.00%> (ø)
swift_code_metrics/_analyzer.py	98.71% <100.00%> (+0.12%)	⬆️
swift_code_metrics/_helpers.py	100.00% <100.00%> (ø)
swift_code_metrics/_metrics.py	100.00% <100.00%> (ø)
swift_code_metrics/_parser.py	95.91% <100.00%> (+0.04%)	⬆️

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