not support multi-path gopath

[wencan]

My team has not switched to go module.
The value of my gopath contains multiple paths, like this:

/Users/user/gohome:/Users/user/project1:/Users/user/project2

I get an error: "Source directory is outside GOPATH", When I execute mockgen under the project.
I have submitted a new Pull Request #435 to solve this problem.

[codyoss]

Accepted, thank you!

[asv]

this patch breaks Bazel based environments.

[codyoss]

@asv Please open a detailed issue with a reproducer.