Add a --prune flag to just filter packages within a name

[jaimergp]

Adds:

  --prune SPEC          Remove the distributions of this package name that do not match the
                        given constraints.

This allows a channel to be created by trimming a package to the required versions only, but leaving the rest of the channel alone. So, if you just want to use python=3.10 in your channel, instead of having to do

... --remove python=3.8 --remove python=3.9 --remove python=3.11 ...

You can do now:

... --prune python=3.10