At the moment the spec semantics still discards the
bindings but allows it syntactically.

Previously it was not possible to bind only a part
of the virtual dependencies that a spec provided.
This commit makes it such that a user can specify
an explicit binding for each virtual dependency
separately.

Packages can now declare virtual dependencies that needs to
be provided together (e.g. "lapack" and "blas" in openblas).
If virtual dependencies are declared as part of the same
"provides" directive, then they will be provided together.

Each edge in the DAG now contains an additional attribute
that is used to track which virtual dependencies are
satisfied by the edge.

This attribute is only added, but not used, in this commit.

This commit permits to parse specs that bind
multiple virtual dependencies to the same spec
e.g. "netlib-scalapack ^lapack,blas=openblas"

As per a discussion with Todd, saying "used together" conveys
better the meaning of this dictionary.

The root spec byte blob is now generated on the fly
for tests.

This should help to better convey the meaning of this attribute
i.e. store somewhere the user requested providers so that we
can honor those requests when concretizing.