Run gogetdoc in the dir containing the file

[andyjack]

My company keeps its go source in a subdir of the main git repo. I typically start vim in the root of the repo. When using K to view the documentation of a keyword in a go file, I get an error (output of gogetdoc) such as:

vim-go: cannot load package containing /my/repo/go/my-daemon/handler/handler.go: go [list -e -json -compiled -test=false -export=false -deps=true -- /my/repo/go/my-daemon/handler]: exit status 1: go: directory go/my-daemon/handler is outside main module

If I pushd go and run vim, the documentation displays as expected, but running vim in this way this complicates some of my non-go editing and plugin setup.

This pr fixes the issue by starting gogetdoc in the dir containing the open file, I presume by making go list able to find the main module.

Thanks!

[bhcleek]

Thank you for contributing.