THRIFT-5019: Duplicate imports from multiple includes from a namespace

If a thrift file includes two files from the same namespace into a
separate file, the generated Go code has duplicate imports for that
namespace. This fixes that.

Client: go
diff --git a/lib/go/test/common/b.thrift b/lib/go/test/common/b.thrift
new file mode 100644
index 0000000..19930e7
--- /dev/null
+++ b/lib/go/test/common/b.thrift
@@ -0,0 +1,5 @@
+namespace go common
+
+struct B {
+    1: optional string b
+}