Move to src/ layout

[tony]

Fixes #406

See also: pypa/packaging.python.org#320

[codecov]

Codecov Report

Merging #407 (d0de168) into master (dc635fe) will increase coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
+ Coverage   72.71%   72.72%   +0.01%     
==========================================
  Files          25       25              
  Lines        2239     2240       +1     
==========================================
+ Hits         1628     1629       +1     
  Misses        611      611              

Impacted Files	Coverage Δ
src/libvcs/cmd/git.py	54.53% <ø> (ø)
src/libvcs/cmd/hg.py	70.12% <ø> (ø)
src/libvcs/cmd/svn.py	52.47% <ø> (ø)
src/libvcs/exc.py	100.00% <ø> (ø)
src/libvcs/parse/base.py	86.84% <ø> (ø)
src/libvcs/parse/git.py	100.00% <ø> (ø)
src/libvcs/parse/hg.py	95.91% <ø> (ø)
src/libvcs/parse/svn.py	93.87% <ø> (ø)
src/libvcs/projects/base.py	92.64% <ø> (ø)
src/libvcs/projects/constants.py	100.00% <ø> (ø)
... and 4 more

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