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/local.md

@@ -0,0 +1,15 @@
+# Local helm repository
+
+A local `helm` repo is a real chart repository: `PUT` a packaged chart to
+`/api/v2/remotes/<repo>/files/<chart>-<version>.tgz` and `index.yaml` is
+generated from each chart's `Chart.yaml` (with sha256 digests) at
+`/api/v1/local/<repo>/index.yaml`; a file not named after its `Chart.yaml`
+name and version is stored but left out of the index. Add the local as a
+member of a helm virtual to serve it alongside remotes.
+
+```bash
+helm package ./mychart
+curl -fsSL --upload-file mychart-0.1.0.tgz \
+  https://artifactapi.k8s.syd1.au.unkin.net/api/v2/remotes/<repo>/files/mychart-0.1.0.tgz
+helm repo add <repo> https://artifactapi.k8s.syd1.au.unkin.net/api/v1/local/<repo>
+```

docs/helm/remote.md

@@ -0,0 +1,17 @@
+# `helm` remote
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `helm` | `index.yaml` | `.tgz` charts |
+
+## Terraform
+
+```hcl
+resource "artifactapi_remote_helm" "jetstack" {
+  name     = "jetstack"
+  base_url = "https://charts.jetstack.io"
+
+  immutable_ttl = 0
+  mutable_ttl   = 3600
+}
+```

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]
+}
+```