goose: use marker functions instead of comments

To avoid making this CL too large, I did not migrate the existing goose
comments through the repository.  This will be addressed in a subsequent
CL.

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

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

@@ -1,8 +1,6 @@
 // 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!"