Allow specifying DNN product (e.g. Evoq Content, Evoq Content Enterprise)

Adds parameters to New-DNNSite, Restore-DNNSite, and Upgrade-DNNSite, to allow
specifying which product to apply when getting install, upgrage, source, and
symbols packages.  Looks in different file paths for the different types of
packages, falling back to community if a certain type of Evoq package cannot be
found.

Fixes #9