Refactor dashboard scuemata to introduce composition slots, allow unspecified panels to validate, and re-enable devenv validation (#38727)

* Re-enable devenv dashboard validation

* Open up dashboard schema composition points

* Introduce composition space at front of scuemata

* Refactor go code to use new composition structure

* Bunch of small cleanups in dashboard.cue

* Enable both base and dist tests of devenv

* Get rid of obsolete CUE loading hacks

* Skip weird failures on these tests

Really don't seem to be testing for what we intend them to be testing
for.

pkg/cmd/grafana-cli/commands/testdata/missing_family.cue

@@ -1,4 +1,4 @@
-package grafanaschema
+package dashboard
 
 import "github.com/grafana/grafana/cue/scuemata"
 
@@ -76,8 +76,3 @@ Dummy: scuemata.#Family & {
 		]
 	]
 }
-
-#Latest: {
-    #Dashboard: Dummy.latest
-    #Panel: Dummy.latest._Panel
-}

pkg/cmd/grafana-cli/commands/testdata/missing_panel.cue

@@ -1,4 +1,4 @@
-package grafanaschema
+package dashboard
 
 import "github.com/grafana/grafana/cue/scuemata"
 
@@ -76,8 +76,3 @@ Family: scuemata.#Family & {
 		]
 	]
 }
-
-#Latest: {
-    #Dashboard: Family.latest
-    #Panel: Family.latest._Panel
-}