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).
- Unsymlinks config/300-imagecache.yaml, since go mod vendor will only
  copy package dependencies.
- Wraps `logger.Sync()` calls in func that explicitly ignores the
  returned error to make golangci-lint happy.
- Replace kodata/VENDOR-LICENSE symlinks with symlink to third_party.

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