USES=mono: improve makenupkg

• add support for v3 of the nuget API
• switch to using v3 for NUGET [1]
• rewrite makenupkg to avoid full feed list [2]
• change fetch group to allow "+" in package name/version

[1] NUGET redirects to the source under the v2 API, however this breaks
makenupkg as curl fails to follow the redirect when getting the header.

[2] 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).