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.
diff --git a/lib/go/test/ConflictNamespaceTestSuperThing.thrift b/lib/go/test/ConflictNamespaceTestSuperThing.thrift
index 2348a62..348d8fb 100644
--- a/lib/go/test/ConflictNamespaceTestSuperThing.thrift
+++ b/lib/go/test/ConflictNamespaceTestSuperThing.thrift
@@ -16,14 +16,26 @@
 # specific language governing permissions and limitations
 # under the License.
 #
+namespace go conflict.super
+
 include "ConflictNamespaceTestA.thrift"
 include "ConflictNamespaceTestB.thrift"
 include "ConflictNamespaceTestC.thrift"
 include "ConflictNamespaceTestD.thrift"
+include "ConflictNamespaceTestE.thrift"
+include "ConflictNamespaceTestF.thrift"
 
 struct SuperThing {
   1: ConflictNamespaceTestA.ThingA thing_a
   2: ConflictNamespaceTestB.ThingB thing_b
   3: ConflictNamespaceTestC.ThingC thing_c
   4: ConflictNamespaceTestD.ThingD thing_d
+  5: ConflictNamespaceTestE.ThingE thing_e
+  6: ConflictNamespaceTestF.ThingF thing_f
+}
+
+// Define an enum to force the import of database/sql/driver
+enum Enum {
+  One = 1
+  Two = 2
 }