Fix comparison of frozen sources.

This was the key line here... it skipped the block if both lists were identical (including sort order) without looking at the sources of either set (because `Gem::Dependency#==` does not consider source in the comparison).

When the dependencies happened to be in a different order than the locked dependencies, then the block _was_ executed. Converting `@dependencies` and `@locked_deps` to sets before comparing ended up causing many more test cases to fail, so this if statement was actually masking other bugs.