Project structure cleanup

[GPMueller]

• All project configuration files are now required to be present, i.e. they cannot be downloaded by clang-build
• more transparent setup stages
• new TargetDescription class which is used to get a dependency graph before creating Target instances and downloading sources

Related to issue #64, as this PR implements a significant part of the described changes.

[GPMueller]

Looking at this reference, I would say we should decide how we handle __str__, __repr__ and identifier for the Project, TargetDescription and Target classes. This also comes into play in the Circle class, where it makes a difference in the way that circular dependencies are printed, for example:

[circular_project.mylib1] -> [circular_project.mylib2] -> [circular_project.mylib1]
or
circular_project.mylib2 -> circular_project.mylib1 -> circular_project.mylib2

In my opinion, it would make sense to have it similar to datetime.datetime, i.e.:

$ print(target.identifier)
project.subproject.target

$print(str(target))
[project.subproject.target]

$print(repr(target))
clang_build.target.Target('project.subproject.target')

[codecov[bot]]

Codecov Report

Merging #108 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   80.47%   80.47%           
=======================================
  Files          16       16           
  Lines        1168     1168           
=======================================
  Hits          940      940           
  Misses        228      228           

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