support in-repo images from the version stream

[jstrachan]

it would be nice to have a syntax for image names to use inside in-repo tekton pipelines/tasks which get the image version from the version stream if it exists

e.g. inside a step...

steps:
- image: versionStream:gcr.io/myrepo/something

or maybe a magic suffix?

steps:
- image: gcr.io/myrepo/something:@versionStream

to indicate that lighthouse should lookup the file gcr.io/myrepo/something.yaml to find the version in the following paths in order, picking the first one:

• images the new local image stream outside of the kpt managed versionStream folder
• versionStream/images(new location in the shared version stream)
• versionStream/docker (current location