split per-type registration points into per-type files

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

docs/goproxy/remote.md

@@ -0,0 +1,5 @@
+# `goproxy` remote
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `goproxy` | `@v/list`, `@latest` | `.info`, `.mod`, `.zip` |