Many packages implement logic at the class level to handle complex dependencies and
conflicts. Others have started using `with when("@1.0"):` blocks since we added that
capability. The loops and other control logic can cause some pure directive logic not to
be removed by our package hashing logic -- and in many cases that's a lot of code that
will cause unnecessary rebuilds.
This commit changes the unparser so that it will descend into these blocks. Specifically:
1. Descend into loops, if statements, and with blocks at the class level.
2. Don't look inside function definitions (in or outside a class).
3. Don't look at nested class definitions (they don't have directives)
4. Add logic to *remove* empty loops/with blocks/if statements if all directives
in them were removed.
This allows our package hash to ignore a lot of pure metadata that it was not ignoring
before, and makes it less sensitive.
In addition, we add `maintainers` and `tags` to the list of metadata attributes that
Spack should remove from packages when constructing canonoical source for a package
hash.
- [x] Make unparser handle if/for/while/with at class level.
- [x] Add tests for control logic removal.
- [x] Add a test to ensure that all packages are not only unparseable, but also
that their canonical source is still compilable. This is a test for
our control logic removal.
- [x] Add another unparse test package that has complex logic.
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