SDK should work outside of `$GOPATH/src`

[estroz]

Feature Request

Is your feature request related to a problem? Please describe.
The SDK depends on GOPATH being set in a users' development/build environment to run certain commands. Specifically CheckAndGetProjectGoPkg() requires $GOPATH to get the project's repo, a value used in several scaffolds and pieces of SDK logic.

Describe the solution you'd like
Go modules, which removes the GOPATH requirement for go projects in general, are now usable by existing and new operator projects through the SDK. The SDK should support the same behaviour.

Note: API packages are assumed to be rooted under $GOPATH by k8s.io/gengo, the parsing library used by controller-tools. The SDK uses controller-tools to generate CRD's. Until this issue is fixed (see this comment) the SDK cannot work outside of $GOPATH.