THRIFT-4797: Go import improvements

This change improves two problems in go code imports:

1. Always rename import the thrift package into "thrift", as we allow
   the user to use a different library to replace the official one from
   the compiler command line, this makes sure that in compiler generated
   go code we can always blindly use "thrift.*".

2. We added auto rename import dedup in d9019fc5a4, but in that change
   for system packages we always use the full import path as the dedup
   identifier, so system package "database/sql/driver" would not be
   detected as a conflict against a thrift go namespace of
   "foo.bar.driver". Use the part after the last "/" in system packages
   as the dedup identifier instead.
7 files changed