When remotely installing a plugin, constraints are used by Packer to
determine which version of a plugin to install.
These constraints can be arbitrarily complex, including operators and
ranges in which to look for valid versions.
However, the versions specified in those constraints should always be
final releases, and not a pre-release since we don't explicitly support
remotely installing pre-releases.
This commit therefore addds checks to make sure these are reported ASAP,
even before the source is contacted to list releases and picking one to
install.
Lucas Bajolet
b4a843c4e2