Add non-standard workspaces to package path list

[DeepAnchor]

Allows guru tools to work with gb, godep, etc

[fatih]

Hi @DeepAnchor

Thanks for your contribution. Can you please give a detail on how guru uses this and what it improves?

[DeepAnchor]

Sure. Guru tools such as implements, describe, callees, callers, etc., rely on being able to find package import paths to perform its analysis. Currently, go#package#Paths() only scans $GOROOT and the default $GOPATH to find these import paths, causing guru commands to fail when projects are located elsewhere with the message "current directory is not inside of a valid GOPATH".

For example when using gb, projects could be located in an arbitrary location outside of $GOPATH and vim-go will fail to recognize them as valid workspaces, causing all guru commands to fail. With this patch, the current workspace will be detected at runtime using go#path#Detect() instead of $GOPATH which will allow guru to find the proper import paths.

[fatih]

Thanks for the clarification @DeepAnchor. I've asked because the function is being used a lot every where else and we still don't have tests (I hope I'll start them soon) to see the global affect I need to be careful while picking stuff.

[DeepAnchor]

No problem, better safe than sorry!