Add SpawnInputExpander helper class to arrange runfiles for spawn strategies

This new class is a combination of SpawnHelper and our internal code; the
plan is to migrate all spawn strategies to the new class. The strict flag
should be enabled by default, but that's a breaking change, so we need to do
it later.

- Use it in SandboxStrategy.
- Add ActionInput.getExecPath to return a PathFragment; this avoids lots of
  back and forth between path fragments and strings.

This is a step towards #1593.

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PiperOrigin-RevId: 150427021
MOS_MIGRATED_REVID=150427021

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