By introducing a new package metadata field, `provider_priority`
(index letter `k`), we can specify default packages to satisfy a
virtual.
If a user wishes to select an alternative provider for the virtual,
a changeset swapping the default provider for the selected provider
will be generated by the dependency resolver.
Instead of the dependency oriented logic, switch to print them
for each package or name needed. Might give a bit more readable
errors now. There's still few corner cases that proper error is
not output, which are cought by the test cases.
William Pitcock