WIP: no review yet. Make code generation not require a GOPATH set

[andrewmains12]

What this PR does / why we need it:

Since m3 is now a go.mod project, it's no longer necessary to be checked out under GOPATH.

This PR makes our code generation reflect that fact. I've switched to use of relative paths in generate.go, which as I understand it is the preferred way of doing things generally.

Note: I haven't yet fixed the protobuf generation--there are a couple more kinks to work out there. This PR handles thrift and mocks.

Fixes #4134