goose: strip vendor from generated import paths

This allows goose to work more gracefully in a vgo setting.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/Vendor/foo/goose.go

@@ -0,0 +1,11 @@
+//+build gooseinject
+
+package main
+
+import (
+	_ "bar"
+)
+
+//goose:use "bar".Message
+
+func injectedMessage() string