Add 'aspect_id' for to identify aspects that has been applied to the target.

Add a concept of aspect_id: an unique string that identifies a
propagating aspect (aspect class + parameters).
This string is designed to be:
- Unique for each aspect
- human-readable for debugging purposes
- not easily parsable.

Skylark API:
- `ctx.aspect_id` returns an identifier of the current aspect inside
  aspect implementation    function
- `Target.aspect_ids` return a list of aspect ids for aspects applied to
  a given Target (https://www.bazel.io/versions/master/docs/skylark/lib/Target.html)

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PiperOrigin-RevId: 141057865
MOS_MIGRATED_REVID=141057865

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