persist files to not generate into go_package as fallback

[iamneal]

when running protoc with the persist plugin, the place to put your persist implementation is decided by first checking the persist.package option, if that does not exist, it checks your go_package option, if that does not exist, it falls back to the package.

tested generating the spanner/basic example with no persist package, but go_package set. it put my generated code in the examples folder, with the package name of examples. It should have put my code in the examples/spanner/basic folder with the package name basic

[iamneal]

This is now fixed on master.