Currently, a large tree artifact cannot benefit from the Merkle tree cache if
it always appears on a nested set together with other (unique per-action)
files.

To improve this, modify SpawnInputExpander to treat the tree as a distinct
node in the input hierarchy that can be cached separately.

Also simplify the cache keys for filesets and runfiles, since the
SpawnInputExpander is a per-build singleton, and this cache is only shared by
actions within a single build.

Progress on bazelbuild#17923.

Closes bazelbuild#17929.

PiperOrigin-RevId: 522039585
Change-Id: Ia4f2603325acfd4400239894214f2884a71d69cf