PEP 735: Draft 2 of Dependency Groups in pyproject.toml (python#3566)

The major changes are as follows:
- remove support for path dependencies (!)
- include a reference implementation
- move 'use cases' into appendix
- rephrase position vis-a-vis Poetry and PDM, and move details
  of those tools into an appendix
- include more example data throughout
- explicitly address the lack of standardization around "reserved
  names" for Dependency Groups in several places

This revision should be shorter as well. Some redundant and wordy
sections have been struck or rewritten.

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Co-authored-by: Brett Cannon <brett@python.org>