Add virtual packages in run into the hash

[wolfv]

That is a relatively recent thing in conda-build.

[wolfv]

Discussion here: conda/conda-build#4606

The idea is that this enables building platform specific noarch packages.

[wolfv]

The way this should be done in our case is that we add the __osx: <matchspec> or other virtual package specifiers to the variant table and then it will be automatically part of the hash. That is also simpler for the implementation.
We can also make it specific to only happen when building noarch packages.