Improve equality and interning semantics for source ref and spec

[hannes-ucsc]

Supersedes #3250

From #6765 (comment):

Two SourceRef instances should be equal if and only if their .id is equal. There should be no two SourceRef instances with equal .id and unequal .spec. I'm unsure if this should be enforced via interning or in the __eq__ override. The enforcement via interning would be deterministic. Detecting a discrepancy in the __eq__ override would be probabilistic. However, the interning involves a lock which could limit concurrency. I also doubt that the memory savings obtained by interning are significant. More thought and/or experimentation needs to go into this.

Two SourceSpec instances should be equal if an only if they're of the same class and and equal in the value of all of their attributes. Instead of having an optional prefix in the SourceSpec base class, there should be a decorator class called SourcePartition with a mandatory prefix attribute, and another attribute called spec for the SourceSpec instance that it decorates. I'm not sure if SourcePartition would be a subclass of SourceSpec or not. If not, the SOURCE_SPEC type parameter to the SourceRef type would not have a bound. But either way, a SourcePartition instance would never be equal to another SourceSpec instance that is not a SourcePartition. That's what would make the .id indirection in the fix for #67651 redundant.