plugindef: Move pluginmeta out of coremodels as standalone thema lineage (#56765)

* Get pluginmeta mostly moved over to pkg/plugins/plugindef

* Remove dead func

* Fix up pfs, use sync.Once in plugindef

* Update to latest thema

* Chase Endec->Codec conversion in Thema

* Comments on slash header gen; use ToSlash

* Also generate JSON schema for plugindef

* Generate JSON Schema as well

* Fix slot loading from kindsys cue decls

* Remove unused vars

* skip generating plugin.schema.json for now

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

pkg/plugins/plugindef/plugindef.go

@@ -0,0 +1,38 @@
+package plugindef
+
+import (
+	"sync"
+
+	"cuelang.org/go/cue/build"
+	"github.com/grafana/grafana/pkg/cuectx"
+	"github.com/grafana/thema"
+)
+
+//go:generate go run gen.go
+
+// PkgName is the name of the CUE package that Grafana will load when looking
+// for kind declarations by a Grafana plugin.
+const PkgName = "grafanaplugin"
+
+func loadInstanceForplugindef() (*build.Instance, error) {
+	return cuectx.LoadGrafanaInstance("pkg/plugins/plugindef", "", nil)
+}
+
+var linonce sync.Once
+var pdlin thema.ConvergentLineage[*PluginDef]
+var pdlinerr error
+
+// Lineage returns the [thema.ConvergentLineage] for plugindef, the canonical
+// specification for Grafana plugin.json files.
+//
+// Unless a custom thema.Runtime is specifically needed, prefer calling this with
+// nil, as a cached lineage will be returned.
+func Lineage(rt *thema.Runtime, opts ...thema.BindOption) (thema.ConvergentLineage[*PluginDef], error) {
+	if len(opts) == 0 && (rt == nil || rt == cuectx.GrafanaThemaRuntime()) {
+		linonce.Do(func() {
+			pdlin, pdlinerr = doLineage(rt)
+		})
+		return pdlin, pdlinerr
+	}
+	return doLineage(rt, opts...)
+}