Setup per project folder metadata and tools

[elcritch]

The goal of this PR is to enable transitioning to a folder-per-package shape from which packages.json is generated. It still supports adding packages directly to packages.json (for now).

This would make adding new packages in the future easier and avoid json clobbering. For now we will need to synchronize pkgs/ <-> packages.json.

To do that, this PR introduces sharded package metadata under pkgs/<first-letter>/<package-name>.json while keeping the existing top-level packages.json workflow compatible for now. E.g. current nimble publish PRs should keep working.

• A new package_index.nim tool now handles splitting, combining, and push-time synchronization between packages.json and pkgs/.
• CI was also reworked around that model. PR validation now uses package_scanner.nim --check-pr --check-urls directly against the git merge base, enforcing that PRs either add new packages or modify existing ones, but not both.
• Push handling moved into merge.yml, which runs only on master, syncs whichever side changed, validates the resulting manifest, and auto-commits the generated counterpart when needed.

[elcritch]

@ringabout and @Araq what do you think of this? And grr, gotta cleanup merge scraps.

[elcritch]

This also adds a ./package_index add and a ./package_index create for new packages. The create prompts for the data. Then ./package_index remove foo which removes a package.

[elcritch]

@ringabout @Araq if we get this merged we can start going to collecting better package metadata!

[hugosenari]

Is a good moment to ask about package namespace?
Ie, instead of per package, have it per owner
docker: org/package
npm: @org/package (org was added to v2 or something)
mvn: groupId=com.github.owner&artifactId=package
nuget: org.package
pypi: package
CPAN: package
cargo: package
PURL: pkg:nimble/org/package@version?args=vals#subpath (but org is optional)