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/plugin.go

@@ -0,0 +1,24 @@
+// Package consts is a test fixture: ID() and Requires() return constants
+// of the same package, of another package in the module and of a package
+// in another module. kiosk reads it with go/parser; it is never built.
+package consts
+
+import (
+	"example.test/sm-consts-plugin/ids"
+	"example.test/sm-other-plugin"
+)
+
+// PluginID is the plugin ID, built from two constants.
+const PluginID = vendor + ".consts"
+
+// Plugin is the fixture plugin.
+type Plugin struct{}
+
+// ID returns PluginID.
+func (*Plugin) ID() string { return PluginID }
+
+// Requires a plugin whose module is not required (acme.translate) and the
+// plugin of the other module.
+func (*Plugin) Requires() []string {
+	return []string{ids.Translate, other.PluginID}
+}