mv variants: packages are now needed only during normalization (#4129)

* mv variants: packages are now needed only during normalization

The relationship among different types of variants have been weakened,
in the sense that now it is permitted to compare MV, SV and BV among
each other. The mechanism that permits this is an implicit conversion
of the variant passed as argument to the type of the variant asking
to execute a constrain, satisfies, etc. operation.

* asbtract variant: added a new type of variant

An abstract variant is like a multi valued variant, but behaves
differently on "satisfies" requests, because it will reply "True"
to requests that **it could** satisfy eventually.

Tests have been modified to reflect the fact that abstract variants
are now what get parsed from expressions like `foo=bar` given by users.

* Removed 'concrete=' and 'normal=' kwargs from Spec.__init__

These two keyword arguments where only used in one test module to force
a Spec to 'appear' concrete. I suspect they are just a leftover from
another refactoring, as now there's the private method '_mark_concrete'
that does essentially the same job. Removed them to reduce a bit the
clutter in Spec.

* Moved yaml related functions from MultiValuedVariant to AbstractVariant.

This is to fix the erros that are occurring in epfl-scitas#73, and that
I can't reproduce locally.