pmc: Fix executing Private Metrics codegen tests

Currently, the Private Metrics codegen tests were siletly skipped due to
a name clash between private_metrics/gen_builders_test and
ukm/gen_builders_test. This patch renames the file to include PMC tests
in the metrics test suite.

This patch also renames codegen.py, because it can also cause tests to fail after enabling them due to a name clash.

Bug: 448819123
Change-Id: I31a718fac4ab50d66efac79ebe1411ced65be0f7
Bypass-Check-License: Moved files around
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7003022
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Jay Zhou <zhouzj@google.com>
Commit-Queue: Kamil Jarosz <kjarosz@google.com>
Cr-Commit-Position: refs/heads/main@{#1525644}
NOKEYCHECK=True
GitOrigin-RevId: 7e5c1e82b744232c1ead28a0331180e502959883