prepare a migration from pluggy hooks to entrypoints

[RonnyPfannschmidt]

as hatch uses pluggy mostly to obtain a list of named plugin classes,
we discovered in duscussion on discord that entrypoints may be better

the example taken for input is

[project.entry-points."hatch.hook.metadata"]
vcs =  "hatch_vcs.metadata_hook:VCSMetadataHook"
[project.entry-points."hatch.hook.version_source"]
vcs =  "hatch_vcs.version_source:VCSVersionSource"
[project.entry-points."hatch.hook.build"]
vcs =  "hatch_vcs.build_hook:VCSBuildHook"

tools to supplement the migration may be necessary

a most practical tool would be a app that finds hooks, finds the class names, then uses their definitions and attributes to pre-register the implementations