When using default modules, this feature will add conflicts to
module-build-macros for every RPM in a buildrequired base module
that overlaps with RPMs in the buildrequired modules. This will
prevent them from being available in the buildroot, and thus
ensure that the RPMs from the buildrequired modules (non-base
modules) are used even if they have a lower NVR.
A base module can set xmd.mbs.default_modules_url, which contains a
URL to a list of modules in the format of name:stream separated by
new lines. When a module buildrequires this base module, the list
of default modules are added as buildrequires of the module automatically
unless there are conflicting streams or the default module is not
in the MBS database.