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/ConflictNamespaceTestE.thrift b/lib/go/test/ConflictNamespaceTestE.thrift
new file mode 100644
index 0000000..07d6c01
--- /dev/null
+++ b/lib/go/test/ConflictNamespaceTestE.thrift
@@ -0,0 +1,23 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements. See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership. The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License. You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+namespace go conflicte.driver
+
+struct ThingE {
+  1: bool value
+}