allow pkgManager as optional

In repos with a single package, it makes less sense to specify a pkgManager. If we only specify the packages array, it currently throws an error. Fix this to make pkgManager optional.

Start by locating the configuration validation for the packages array and pkgManager option. Reproduce the error with a single-package repository that specifies only packages, then update the behavior so that configuration is accepted without pkgManager and verify the existing package-management checks still pass.

Written by the indexing model from the issue text.