USES=mono: improve makenupkg
- add support for v3 of the nuget API
- switch to using v3 for NUGET 
- rewrite makenupkg to avoid full feed list 
- change fetch group to allow "+" in package name/version
 NUGET redirects to the source under the v2 API, however this breaks
makenupkg as curl fails to follow the redirect when getting the header.
 Previously, makenupkg got the complete list of packages (and versions)
a feed. However, for some feeds this takes a very really long time (slow
server response rate). To avoid this, rather check if the package is
available (without downloading it).