pageserver: respect has_relmap_file in collect_keyspace

[jcsp]

Problem

Rarely, a dbdir entry can exist with no relmap_file_key data. This causes compaction to fail, because it assumes that if the database exists, then so does the relmap file.

Basebackup already handled this using a boolean to record whether such a key exists, but collect_keyspace didn't.

Closes: #8237

Summary of changes

• Respect the flag for whether a relfilemap exists in collect_keyspace
• The reproducer for this issue will merge separately in tests: use smaller layers in test_pg_regress, do compaction after tests #8232

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (6938 of 21281 functions)
• lines: 50.0% (54545 of 109057 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9717551 at 2024-07-08T14:16:28.455Z :recycle:}