Vendor dependencies when running code generators.

[mattysweeps]

This PR uses go mod vendor to vendor code-generation dependencies, giving us more reliability.

Goal were:

1. Use the same version of code-generator that knative uses.
2. Because of 1., remove all sed hacks when generating code
3. (Still to test) fix our code incompatibility with watchers

Solution was:

1. enforce go mod vendor by using replace directives in go.mod to bin k8s.io dependencies as close as we could to k8s version 1.12.9: the version used by knative/pkg and knative/serving.
2. Update our scripts to utilize the vendor direcotry
3. Because go mod vendor does not down;pad scripts, we download the code generation scripts manually. This makes me sad.

Other considerations:

1. Dep cannot be used because we need 2 different versions of knative/pkg, because of our dependency on knative build. Once we move to tekton we might be able to switch to dep, making us more consistent with knative.

Side effects were:

1. the go mod cache cannot be used with vendored dependencies, causing a massive slowdown in the generation script.

[josephlewis42]

LGTM