The main non-SCL, ISC-provided BIND package is called "isc-bind".  Its
version number reflects the BIND version it contains.

The Software Collection metapackage is also called "isc-bind" and has
its version number set to 1.  If it was put in the same repository as
the non-SCL package, it would not be considered to be an update for the
non-SCL "isc-bind" package.

Since we want to migrate existing users of non-SCL packages to SCL
packages, the Software Collection metapackage needs to have a version
number higher than the non-SCL, ISC-provided BIND package.  The arguably
most elegant way to achieve that is to set an Epoch for the Software
Collection metapackage.