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/testdata/consts/ids/ids.go

@@ -0,0 +1,5 @@
+// Package ids holds plugin ID constants for the consts fixture.
+package ids
+
+// Translate is a plugin ID no module of the fixture provides.
+const Translate = "acme.translate"