feat(kiosk): resolve constant plugin IDs, overrides and the CheckT helper

- kiosk.ReadIdentity: ID()/Requires() as literals or constants across packages and modules, read with go/parser, never run
- plugin minimums from go.mod requires; v0.0.0 is an error, a pseudo-version warns
- kiosk.Validate, kiosk.CheckT (no testing import)
- plugin:add reads IDs through kiosk.ReadIdentity, so const-ID plugins register

modules/kiosk/checkt_test.go

@@ -0,0 +1,14 @@
+package kiosk_test
+
+import (
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/kiosk"
+)
+
+// TestPluginManifest is the test a plugin repository adds next to its
+// plugin.go, with "." as the directory. Here it guards the committed
+// manifest of the literal fixture.
+func TestPluginManifest(t *testing.T) {
+	kiosk.CheckT(t, "testdata/literal")
+}