Disambiguate generated classes based on source statement

MDSAL-502 describes a rather ugly case where the original models themselves are generated and use a weird mix of naming.

While the complete solution still requires a bijective mapping, that ends up going to Unicode-land for a solution, which will invariably make things very unreadable.

We already need to deal with YANG statement namespace overlap, as we are mapping multiple constructs into Java FQCN namespace: identity, feature, container, etc., so we are already equipped to deal with:

module foo {
  identity foo;
  feature foo;
  container foo;
}

This disambiguation stops at schema tree statements vs. others, hence we do not handle this:

module foo {
  anydata foo.bar;
  anyxml foo-bar;
  container Foo.Bar;
  list Foo-Bar;
  leaf foo.Bar;
  leaf-list foo-Bar;
  notification Foo.bar;
  rpc Foo-bar;
}

and some other cases we generate Java constructs.

We can improve the situation by generalizing over 'class source statement' and assign a $XX suffix, similar to $F, $G, $I, and $YD we already assign.

There is already a FIXME to assign two-letter suffixes for a number of these, let's bring this in.

The advantage of doing this renaming before MDSAL-502 is that it involves '$', which is not nice, but it does not involve weird Unicode characters (which works, but working with it is a ... memorable .. experience).