fix(uv): Transition build deps of whls back to libs

[arrdem]

Fix a bug when a requirement with no bdists is depended on as a whl rather than as a py_library which manifests as build not found errors which are confusing because in lib mode, the requirement will build just fine.

In order to facilitate future pex/sci-e/zipapp support, one of the features of the uv machinery is that the requirements graph can be configured so that dependencies occur either as installed library trees OR as pre-install wheel files. This solves a persistent issue with zipapp-like things of needing to re-zip/re-whl files which have already been installed, leading to among other things useless I/O and whls inside zipapps which may differ from the distribution whls.

This bug arises because once the transition flag is set from lib (the default) to whl by a py_whl_library as part of a venv's hub, that flag is never reset back to lib which needs to happen in order for sdist builds with dependencies to succeed. Sdist builds require that dependencies be installed, whl files do not suffice.

Ideally we'd use Fabian's resettable targets from with_cfg.bzl, however using his resetters creates problems. So do the lame but obviously correct thing of handwriting equivalent transitions.

Changes are visible to end-users: no

Test plan

Repairs building all targets under a uv-defined hub, which was previously broken.

❯ bazel build @pypi//... 
INFO: Analyzed 218 targets (71 packages loaded, 1481 targets configured).
INFO: Found 218 targets...
Target @@+uv+venv__pypi__default//pytest/entrypoints:py.test up-to-date:
  bazel-bin/external/+uv+venv__pypi__default/pytest/entrypoints/py.test
Target @@+uv+venv__pypi__default//pytest/entrypoints:pytest up-to-date:
  bazel-bin/external/+uv+venv__pypi__default/pytest/entrypoints/pytest
INFO: Elapsed time: 0.824s, Critical Path: 0.11s
INFO: 15 processes: 13 internal, 2 linux-sandbox.
INFO: Build completed successfully, 15 total actions

[aspect-workflows]

test-os:linux-bzl:8 (Test)

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test-os:linux-bzl:latest (Test)

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38 tests (100.0%) were fully cached saving 41s.

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test-os:linux-bzl:8 (Test)

e2e

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test-os:linux-bzl:latest (Test)

e2e

All tests were cache hits

14 tests (100.0%) were fully cached saving 11s.

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test-os:linux-bzl:8 (Test)

examples/uv_pip_compile

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1 test (100.0%) was fully cached saving 335ms.

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test-os:linux-bzl:latest (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 335ms.

[dzbarsky]

i don't see this string used anywhere, is this doing something very magical? mind leaving a comment explaining?

[arrdem]

This is #deadwood, will remove.