Make dangling symlinks in external repositories work to some degree.

This seems to be the least insane approach within the following boundaries:

- Skyframe apparently doesn't allow GlobFunction to recover if FileFunction had already raised an exception that Skyframe knows about (this is somewhat surprising)
- I didn't want to change FileFunction not to throw an exception for dangling symlinks because this part of the code is scary
- I didn't want to revert to Skyframe-based symlink resolution for symlinks in immutable directories because that would be a performance hit
- I didn't want to write yet another symlink resolver and the two existing ones (FileSystem#resolveSymlinks() and and FileFunction#getSymlinkTargetRootedPath()) don't work: the former cannot resolve just one level of symlinks and the latter cannot do its job without adding Skyframe dependencies

I had to put in a placeholder value for realRootedPath when the FileValue represents a dangling symlink, because FileStateValue.create() relies on the symlink target being different than the symlink itself.


RELNOTES:

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