Allow provider-wide naming config

[imjasonh]

ko provides a few ways to name images it builds. Given the importpath foo.bar/test:

• the default, producing $KO_DOCKER_REPO/test-<md5>, aiming for compatibility with the most registries (Dockerhub mainly)
• --preserve-import-paths, producing $KO_DOCKER_REPO/foo.bar/test
• --base, producing $KO_DOCKER_REPO/test, which leads to trouble if there is a foo.bar/test and a foo.bar/baz/test.
• --bare, producing $KO_DOCKER_REPO exactly (best for ko build, less good for ko resolve)

In tf-ko, ko_build currently defaults to --preserve-import-paths behavior, and ko_resolve defaults to the default (MD5), which will lead to confusion.

We should make the naming config a provider-wide default, since that's normally what folks want to do anyway. We can take this opportunity to make the default match ko, or use the more correct and ergonomic --preserve-import-paths the default, since it's easier to configure broad defaults in tf-ko.