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/helm/virtual.md

@@ -0,0 +1,9 @@
+# `helm` virtual
+
+```hcl
+resource "artifactapi_virtual" "helm" {
+  name         = "helm"
+  package_type = "helm"
+  members      = [artifactapi_remote_helm.jetstack.name]
+}
+```