refactor registry module to return URLs rather than making the requests

[dmeliza]

The registry module has a bunch of functions that are essentially just wrappers around calls to requests with URLs. There is a lot of duplicated code, but the bigger issue is that it locks in a dependency on the requests module that might be nice to bypass in some circumstances. The calls to requests should be moved out to the core module, while registry just provides functions for constructing URLs and query strings