When computing the solution set, if a gem is already loaded, make sure
to use the specification of the loaded one instead of the first
available as otherwise there is a risk that when multiple matches are
available the specification for the wrong version may be picked.
When this happens an error message will be triggered that looks like
can't activate json-2.3.0, already activated json-2.5.1
This can occur for distribution packaged vagrants as well as installs
for development purposes where the ruby install may contain a default
gem spec of an older version than is needed.
Fixes: #12521Fixes: vagrant-libvirt/vagrant-libvirt#1390
0c6d6d8e9d