Support proxy and local publishing of the same package

[isc-tleavitt]

Motivating use case:
Internal package registry used prior to publishing to Open Exchange. I want to be able to publish (say) isc-codetidy 1.1.1-alpha.1 locally, for use by registry clients that accept prereleases (see #91), then have it supplanted by the real isc-codetidy 1.1.1 once that's published to the Open Exchange / pm.community.intersystems.com.