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/vendor/bar/bar.go

@@ -0,0 +1,9 @@
+// Package bar is the vendored copy of bar which contains the real provider.
+package bar
+
+//goose:provide Message
+
+// ProvideMessage provides a friendly user greeting.
+func ProvideMessage() string {
+	return "Hello, World!"
+}