pageserver: detach ancestor should copy files from unshard portion of the timeline

[skyzh]

Description

See INC-320 (slack channel).

Steps to reproduce

1. have some writes to a timeline that creates some layer files
2. do a shard split, keeping references to those layer files
3. create a branch (either before or after shard split, it doesn't matter)
4. do a detach ancestor for that child branch

Expected result

no errors

Actual result

There will be errors in the detach ancestor step that copies layers in S3. With a test environment that uses LocalFs (reproducer from #9669):

WARN request{method=PUT path=/v1/tenant/a10c21187d817bd102ed45865d27d4d8-0304/timeline/30a22fa5bb9b4057597733fedc7ea2a3/detach_ancestor request_id=acce4b63-bba1-4cee-bfc1-09f3c20a74c5}:detach_ancestor{tenant_id=a10c21187d817bd102ed45865d27d4d8 shard_id=0304 timeline_id=30a22fa5bb9b4057597733fedc7ea2a3}: copy timeline layer failed, will retry (attempt 6): copy layer tenants/a10c21187d817bd102ed45865d27d4d8-0304/timelines/433ed006163e534f6cc8bf2aa2f4d549/000000067F00000001000004E70000000020-000000067F00000001000004E70000000030__00000000014F2918-00000001 to tenants/a10c21187d817bd102ed45865d27d4d8-0304/timelines/30a22fa5bb9b4057597733fedc7ea2a3/000000067F00000001000004E70000000020-000000067F00000001000004E70000000030__00000000014F2918-00000001: Failed to copy file [...]: No such file or directory (os error 2)

With S3, The "Failed to copy file" part is like service error: unhandled error (NoSuchKey): Error { code: "NoSuchKey", message: "The specified key does not exist.", [...] }.