Clean up ExecutionRequirements

- remove BaseSpawn.Local; instead, all callers pass in the full set of
  execution requirements they want to set
- disable caching and sandboxing for the symlink tree action - it does not
  declare outputs, so it can't be cached or sandboxed (fixes #4041)
- centralize the existing execution requirements in the ExecutionRequirements
  class
- centralize checking for execution requirements in the Spawn class
  (it's possible that we may need a more decentralized, extensible design in
  the future, but for now having them in a single place is simple and
  effective)
- update the documentation
- forward the relevant tags to execution requirements in TargetUtils (progress
  on #3960)
- this also contributes to #4153

PiperOrigin-RevId: 177288598

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