pageserver: local layer path followups

[jcsp]

• Rename "filename" types which no longer map directly to a filename (LayerFileName -> LayerName)
• Add a -v1- part to local layer paths to smooth the path to future updates (we anticipate a -v2- that uses checksums later)
• Rename methods that refer to the string-ized version of a LayerName to no longer be called "filename"
• Refactor reconcile() function to use a LocalLayerFileMetadata type that includes the local path, rather than carrying local path separately in a tuple and unwrap()'ing it later.

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]

3024 tests run: 2892 passed, 0 failed, 132 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Postgres 15

• test_partial_evict_tenant[relative_equal]: release

Postgres 14

• test_statvfs_pressure_usage: debug

Code coverage* (full report)

• functions: 31.4% (6313 of 20124 functions)
• lines: 47.3% (47580 of 100657 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a1d43d8 at 2024-05-08T16:58:12.295Z :recycle:}