Split per-type registration points into per-type files (#150)

Parallel package-type PRs conflict because every type edits the same shared lines (README tables, provider.go, server.go imports, the package type list, UI maps, the e2e caching table). A new type now only adds files.

- move per-type README sections into docs/<type>/{remote,local,virtual}.md
- split provider.go capability and metadata types into cap_*.go and metadata_*.go
- import providers via internal/provider/all, one file per type
- declare each package type in package_type_<type>.go, registered via init()
- split the e2e caching cases and UI colours/snippets into per-type files

Reviewed-on: #150
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/puppet/local.md

@@ -0,0 +1,14 @@
+# Local puppet forge
+
+A local `puppet` repo is a Forge: `PUT` a module tarball to
+`/api/v2/remotes/<repo>/files/<owner>-<module>-<version>.tar.gz` and the Forge
+v3 API (`/v3/modules`, `/v3/releases`, `/v3/files`) is served from each
+release's `metadata.json` at `/api/v1/local/<repo>`. Versions are immutable.
+
+```bash
+puppet module build ./hello
+curl -fsSL --upload-file pkg/unkin-hello-0.1.0.tar.gz \
+  https://artifactapi.example.com/api/v2/remotes/<repo>/files/unkin-hello-0.1.0.tar.gz
+puppet module install unkin-hello \
+  --module_repository https://artifactapi.example.com/api/v1/local/<repo>
+```

docs/puppet/remote.md

@@ -0,0 +1,7 @@
+# `puppet` remote
+
+Shared classification, TTL and eviction rules: [caching](../common/caching.md).
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `puppet` | `v3/modules/*`, `v3/releases*` | `.tar.gz` |