Implementation of all-matches functionality + tests

When performing an *aptly snapshot pull*, users might list dependency
versions that can potentially match multiple packages in the source
snapshot. However, the current implementation of the 'snapshot pull'
command only allows one package to be pulled from a snapshot at a time
for a given dependency.

The newly implemented all-matches flag allows users to pull all the
matching packages from a source snapshot, provided that they satisfy the
version requirements indicated by the dependencies.

The all-matches flag defaults to false and only produces the described
behaviour when it is explicitly set to true.