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/other/plugin.go

@@ -0,0 +1,15 @@
+// Package other is a test fixture: a plugin in its own module that the
+// consts fixture requires. It is never built.
+package other
+
+// PluginID is the plugin ID.
+const PluginID = "acme.other"
+
+// Plugin is the fixture plugin.
+type Plugin struct{}
+
+// ID returns PluginID.
+func (*Plugin) ID() string { return PluginID }
+
+// Requires returns no plugins.
+func (*Plugin) Requires() []string { return []string{} }