Remove Path from Location, ParserInputSource and bunch of other low-level classes.

This makes the code cleaner because a lot of places never read the file and thus never needed a Path in the first place. I got to this change in a bit convoluted way:

- I wanted the default tools in Android rules to point to //external:
- I wanted to make sure that that doesn't cause an error is no Android rules are built, thus I had to add some binding for them in the default WORKSPACE file
- I wanted the Android rules not to depend on Bazel core with an eye towards eventually moving them to a separate jar / Skylark code
- The default WORKSPACE file is currently composed from files extracted by the Bazel launcher which would make the Android rules depend on a very core mechanism
- I couldn't simply pass in jdk.WORKSPACE as a String because Location, ParserInputSource and a bunch of other things needed a Path, which a simple string doesn't have.

Thus, this change.

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MOS_MIGRATED_REVID=95828839