Fix smart package completion

[uudashr]

Some people might want to test the main package with the existence of main.go.
On the _test.go file, it will suggest main as package name instead of publisher_test or publisher.

Dir structure example:

pubsub
- cmd
   - subscriber
      - main.go
   - publisher
      - main.go
      - main_test.go <-- package main
- doc.go
- pubsub.go

Q: Why not suggest main_test?
A: Main package are not going to be reused by others, all function might going to be non-exported. So the test package not will use the non _test pattern.

[uudashr]

@ramya-rao-a Should done, but still not sure why it's failed on tip version

[ramya-rao-a]

Thanks @uudashr! Sorry for the delay, work has been very busy

[ramya-rao-a]

This change is now out in the latest update to the Go extension (0.6.79)