Ruby: Two fixes for private methods

[hvitved]

1. ModuleBase::get(A)Method did not work for methods directly annotated with private.
2. Call graph construction did not take into account that private methods can be overridden.

Commit-by-commit review is suggested.

I sampled a couple of new call graph edges for each project, and they all looked legit.

[aibaars]

Why is this distinction important? Both m and nested are instance methods of M. The only difference is that nested only exists after m has been called at leas once.

[hvitved]

It was not clear to me whether such conditional methods (being conditional on other methods being called) should now be included. I thought it would be safer to not include them in this PR, and then we can reconsider later (in fact, I already have a PR lined up with some call graph tests for nested methods).

[aibaars]

True, but you are including conditional methods like:

if rails.version < 6
   def x; end
end

[aibaars]

I'm fine with excluding nested method for now. Just wanted to make sure it was for the right reason.

[hvitved]

True, but you are including conditional methods like

Indeed. Let's add all nested methods in a follow-up PR.

[aibaars]

Looks good to me. One question about "nested" methods,