Go: Extract locations of successfully extracted files

Switch the successfully extracted files query to the `location, message` results format so that we get rich location information when exporting the results of this query to SARIF.  Previously the query used the `message` results format, which meant the interpreted results lacked a location.

Author: henrymercer

go/ql/src/Diagnostics/SuccessfullyExtractedFiles.ql

@@ -9,5 +9,6 @@
 import go
 
 from File f
-where not exists(Error e | e.getFile() = f)
-select f.getRelativePath()
+where not exists(Error e | e.getFile() = f) and
+  exists(f.getRelativePath())
+select f, ""