Add Go module support for pipelines.

This enables Go modules and removes the vendor directory from the
repository.

NOTE: highly recommend viewing this change with
`git diff master -- ':!third_party' ':!vendor'`

Notable changes:
- Removes use of dep in favor of Go modules.
- Removes vendor directory. As a result, switches license notice
  generation to go-license.
- Forks code generation scripts from respective k8s/knative sources to
  allow module aware imports. (I want to fix this upstream sometime in
  the future, but for now this is as best as we can do).

This should not result in any change of functionality.

Fixes #1315, #1538

.gitignore

@@ -17,7 +17,6 @@
 *.dll
 
 # Fortran module files
-*.mod
 *.smod
 
 # Compiled Static libraries