[FIRRTL] Support alternative base paths in LowerClasses. #6817
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We previously had a constraint that paths are targeting entities in the same owning module as the path. This allowed us to assume that a single base path can be created for each owning module, and used in all paths. However, we have transforms that extract entities targeted by paths outside the paths' owning module.
To support this, we can instead plumb through base paths created higher up in the instance graph down to the paths that need them. When a path is detected outside the owning module, we find the module in which the entity is instantiated and pass through its base path to the paths that reference the entity.
Most of the work here is just plumbing, and the PathInfoTable has been extended with new data structures and functions to support this.