Projects the depend on the CLI currently depend on core. This should not
always be the case. The EnvironmentAwareCommand will remain in :core,
but the rest of the CLI components have been moved into their own
subproject of :core, :core:cli.
The plugin cli currently resides inside the elasticsearch jar. This
commit moves it into a plugin-cli jar. This is change alone is a no-op;
it does not change anything about what is loaded at runtime. But it will
allow easier testing (with fixtures in the future to test ES or maven
installation), as well as eventually not loading these classes when
starting elasticsearch.