switch to modulegraph 2 - #5808

This adds tests for names available as public symbols
in the modulegraph2 namespace.

This also removes all incomplete and untested symbols
from the public API (will be added back when I'm
happy about the implementation)

Including full tests. The code doesn't use ast.NodeVisitor to avoid
running out of stack space.

The tests have 100% test coverage according to coverage.py,
but I haven't checked yet if there is sufficent testing.

One thing I don't quite like is a spurious return statement that's
needed to keep coverage.py happy.

I'm not happy about the naming in modulegraph._depproc, and
the global function '_process_namelist' (this should be a method
given its close interaction with ModuleGraph)

There's still a test failure in a test method that
was hidden by a test with the same name, I haven't
investigated yet what's going on here.

…ulegraph2

…into modulegraph2

…into modulegraph2

…into modulegraph2