Support $GOPATH with multiple directories

[tinhtruong]

I use gb to manage dependencies and I setup my $GOPATH to include multiple directories. When I run gofresh, it seems that gofresh only check the first directory in the GOPATH for a package. It would be great if gofresh support this use case. Thanks.

[dmitshur]

Relevant definition of GOPATH from https://golang.org/cmd/go/#hdr-GOPATH_environment_variable:

The GOPATH environment variable lists places to look for Go code. On Unix, the value is a colon-separated string. On Windows, the value is a semicolon-separated string. On Plan 9, the value is a list.