Improve graph submission time for P2P rechunking by avoiding unpack recursion into indices

[fjetter]

Closes dask/dask#11162

This is a bit annoying but it is very effective. It reduces the time to unpack the graph tasks from about 8min down to 2s

cc @dcherian @hendrikmakait

[dcherian]

hehe cool

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    29 files  ± 0      29 suites  ±0   11h 10m 42s ⏱️ + 10m 29s
 4 057 tests + 1   3 953 ✅  -  3     97 💤 ±0  7 ❌ +4 
55 883 runs  +14  53 712 ✅ +10  2 163 💤 ±0  8 ❌ +4 

For more details on these failures, see this check.

Results for commit a571fc4. ± Comparison against base commit 7cbfc4d.

[hendrikmakait]

Thanks, @fjetter!