Rework reparenting

[tristanlatr]

Rework reparenting in post-processing. It ensures that for all time of visiting the ASTs, the model represent the real code structure, since no reparenting is done before post processing.

This patch changes some of the core pydoctor logic, so it should be carefully reviewed.

This changes will help fixing #295 as well as improve our capacity to fix #184 and other all related problems. Together with #723 it will finally fix #295.

Introduces the Module.imports attribute holding a list of resolved imports (like ast.alias but with full module names), this is required by the re-exporting process since only imported names should be reparented.

Introduces the System.modules attribute that is a dict from the module full names to the Module instances. This dict is not changed during reparenting. It's populated when the modules are added to the system. This behaviour better matches the python import system and makes us differentiate a module vs an object with the same fullname as the module in the parent package __init__.py for instance.

This PR should not introduce any changes in behaviour, except it fixes a bug in reparenting process when there as duplicates (see new tests).

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (1a7d052) 91.98% compared to head (416d1d0) 92.77%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #737      +/-   ##
==========================================
+ Coverage   91.98%   92.77%   +0.78%     
==========================================
  Files          47       47              
  Lines        8147     8245      +98     
  Branches     1780     1976     +196     
==========================================
+ Hits         7494     7649     +155     
+ Misses        380      340      -40     
+ Partials      273      256      -17     

Files	Coverage Δ
pydoctor/model.py	94.88% <100.00%> (+1.90%)	⬆️
pydoctor/astbuilder.py	96.82% <91.95%> (+1.63%)	⬆️

... and 23 files with indirect coverage changes

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[tristanlatr]

I'll close this PR since it makes more sens to merge an actual fix that builds on top of this PR.