A large customer has requested the ability to limit which packages are imported
into satellite channels during the import process.  The two prime cases:

1. Prevent a package from being imported into a channel at time of initial
channel import.  For example, specify various games packages and prevent them
from being imported into the 7.3 base channel.

2. Prevent a new package version from being imported into a channel at time of
incremental channel import.  For example, refusing to accept package foo-1.3.1
becase foo-1.3.0 is important to their environment.

This request comes with the very important, and understood, caveat that this
large customer will have to deal with dependency issues that arise from
eliminating packages from the base channel.