Currently, the skipRegistry value in configuration files is ignored,
the final value is either the one the user passed on the command line,
which is the intended behavior, or the value `none` if the user didn't
pass a --skip-registry argument.

This commit fixes this by introducing the new value `default_` to the
SkippackageSuppliers enum to know when the user didn't pass the
skip-registry argument and the value should be taken from the
configuration files.

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>

The latter is more accurate, as we can have system settings
of type UserConfiguration.
This is also a push towards more consistent naming within dub,
as configuration is primarily used for package recipe 'configuration' field.

By moving it to its own module, we can ensure it's almost a leaf module.
We can also extend unittests to check different scenarios
without relying on integration test.