Add rez-build '--local-only' arg

Add an option to only see local packages, if a given package family has any local packages available.

Currently if you have local 'foo-1.0.0' available, but released 'foo-1.0.1' is also visible, then foo-1.0.1 is still taken as priority, even if you're currently developing foo and didn't expect there to be a higher-versioned released package.

The option should take nargs, in case you only want to enable it for specific packages.