Respect skipRegistry value from configuration files ...

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>

Revert "Move data part of dub.platform to dub.data" ...

This reverts commit a0d80e8.
The approach might be wrong headed and will be reverted until
a more comprehensive diff can be submitted.

Move data part of dub.platform to dub.data ...

We want to separate the data part, which is exposed to the PackageRecipe,
from the actual platform check code, which is used when building.

Rename UserConfiguration to Settings ...

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.

Move user settings to dub.data.settings ...

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.