solver: do not auto select virtual-only package with one provider

This reverts most of 0dcbd933 which allowed automatic selection
of package with a "virtual provides" having only one provider.
While convenient, it creates problems if multiple versions of the
same package exist, or if in future other providers would be added
to one of the repositories. This restore the original behaviour,
and improve the error message to tell the user to mention one of
the providers explicitly.

fixes #10810

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