Support for namespace packages

[tristanlatr]

Fixes #343
Related to #824

This includes support for PEP 420 as well as old school declaration of namespace packages.

[codecov]

Codecov Report

Attention: Patch coverage is 98.19820% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.03%. Comparing base (02063d3) to head (a297e5f).

Files with missing lines	Patch %	Lines
pydoctor/astutils.py	96.42%	0 Missing and 2 partials ⚠️
pydoctor/epydoc2stan.py	90.00%	0 Missing and 1 partial ⚠️
pydoctor/model.py	98.96%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #871      +/-   ##
==========================================
+ Coverage   92.79%   93.03%   +0.23%     
==========================================
  Files          47       47              
  Lines        8468     8610     +142     
  Branches     1550     1578      +28     
==========================================
+ Hits         7858     8010     +152     
+ Misses        350      339      -11     
- Partials      260      261       +1     

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[tristanlatr]

This was dead code :/

[github-actions]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅

[tristanlatr]

Supporting namespace packages is more or less useful if we don’t fix #870. But that can come in a second phase.

[tristanlatr]

Caching the exception instance is not a good idea because if cycles with locals.
Instead we should cache the exception type and args and re-raise another instance of it when asked again.