Support new Curse URLs with non-numeric IDs

[HebaruSan]

Background

The format of Curse API URLs changed recently, but some old URLs are still supported; see the discussion in KSP-CKAN/NetKAN#6608 for details.

Old format:

• https://api.cfwidget.com/project/220602

New format:

• https://api.cfwidget.com/kerbal/ksp-mods/photonsail

Any old URL that had been accessed before March 2018 still works thanks to caching internal to the API, but any newly added mod must use the new format.

Problem

Netkan fails on mods added to curse after March 2018. See KSP-CKAN/NetKAN#6608 for an example; PhotonSailor can't be indexed without updating our Curse API logic.

(UPDATE: We switched that mod to SpaceDock instead, so the urgency is lower, but the problem would still affect future new mods hosted only on Curse.)

Changes

Now the id part of the #/ckan/curse/id kref string may contain either a number or a non-number. If it's a number, the old format URL is used for backwards compatibility with existing metadata. If it's not a number, then the new format is used.

In addition, the Netkan log message that prints the API URL being accessed is promoted from debug to info, so it can be seen when --verbose is enabled.

A test is added to exercise the new format.

[Olympic1]

LGTM, don't notice any problems