Consider using native go tool when resolving locations

[mokiat]

It seems that this is the preferred approach when dealing with go files / packages / locations, as can be seen discussed in the following issue:
golang/go#26504

It needs to be checked whether the go tool provides commands for all resolutions we make in the code.

Potential implications:

• The performance might drop a bit
• The tool will need to parse and rely on go output
• The go tool would need to be present on PATH (though would anyone ever use gostub if they haven't set up their go environment?)

[mokiat]

P.S. An alternative is to migrate to the following package:
https://godoc.org/golang.org/x/tools/go/packages

This has already been done partially on the bump_go_11 branch:
https://github.com/mokiat/gostub/tree/bump_go_11