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Improve source root stripping performance. (#10533)

Computing source roots for many files is expensive in a cold run,
largely due to engine overhead.

We previously worked around this by using the target address
as a "representative path" for all files in the target, knowing
that they are all under the same source root.

However with per-file subtargets this achieves nothing.

This change replaces the "representative path" mechanism
with an implicit one that groups files by the dir they're in,
since such files definitely have the same source root.

A synthetic benchmark showed a 5x speedup with this in place.

Note that this change means that we can strip source roots
even after merging snapshots from multiple targets, since
we no longer need to know about the originating targets
to utilize the performance hack.

A future change will leverage this to simplify our parallel
stripped/unstripped code paths.

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