Don't require go.mod to be in top directory of workspace.

[jimgrier]

If the top level directory of a workspace is not where the go.mod file for a package resides, the extension gets confused, flagging imports as errors, etc. Our build layout often has non-Go files of interest in the directories above the Go code, and being able to open at that level would be more convenient.

[stamblerre]

@jimgrier: Are you working with multiple Go modules or just a single module? If it is a single module, it does not have to be at the workspace root, but if you are working with multiple modules, there are other challenges. This documentation should explain the details and current available options.