make more use of YAML anchors in dependencies.yaml

[jameslamb]

Description

Contributes to rapidsai/build-planning#33

Follow-up to #16299

This proposes some simplifications to dependencies.yaml. It's not intended to change any behavior.

• more use of YAML anchors for requirements that are intended to be identical to each other
• eliminating the pylibcudf_build_dep dependency group that was introduced in Setup pylibcudf package #16299, in favor of just tracking the pylibcudf build dependency alongside cudf's rmm build dependency in the existing build_python_cudf group
  • (sorry I'd missed that in the review on Setup pylibcudf package #16299)

I found myself starting to make similar changes in the PR breaking up these packages into more (splitting out a libcudf in #15483) and thought they'd be better as a standalone PR.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[jameslamb]

/merge