Fix GOPATH handling

[gdamore]

So $GOPATH handing is wrong on several levels.

First, $GOPATH can consist of multiple directories. So our attempt to look up a file using it as a single directory is broken.

Second, not everyone installs from source.

So probably the best place for this is in $HOME. Or maybe we can figure out a way to locate "library" or support files for Go? I'm not sure if such a thing exists.

[gdamore]

So I've added code to look in $HOME/.tcelldb, as well as the full $GOPATH directory list and $TCELLDB.