add guru support refer folder outside gopath

[hui zhang]

"Find All References" does not find code in other packages outside GOPATH    in vscode

https://github.com/microsoft/vscode-go/issues/2236

https://github.com/Microsoft/vscode-go/issues/1602

https://github.com/Microsoft/vscode-go/issues/1473

the root cause is that guru did not support that.

as guru is part of the official tool.

pls add feature, 

guru support refer folder outside gopath.

the folder could be project root path