[back compat] some constants and code is missing from maven-embedder

[laeubi]

New feature, improvement proposal

MavenCli is deprecated, but to ease migration it would be good to have it as "backward compatible" as possible.

Currently it misses some public static constants (e.g. LOCAL_REPO_PROPERTY).

Beside that the org.apache.maven.cli.internal.extension is missing now, instead of deleting code it would be better to mark it deprecated as well.