Matrix expansion with pinned subpackages

[wolfv]

We sometimes have a situation where a package is exactly pinning a subpackage and the pinned subpackage has multiple variants. If the matrix is not the same for the pacakge & subpackage, only one package might be created.

E.g.

conda_build_config.yaml

A:
- 1.0
- 2.0

outputs:
   package:
      name: x1
   dependencies:
      run:
         - A
    package:
       name: x2
     dependencies:
        run:
            - {{ pin_subpackage('x1', exact=True) }}

We should either warn or automatically expand the matrix for x2 with all variants of x1.

[Tobias-Fischer]

Nice! Real life example where this has bitten us: conda-forge/bullet-feedstock#45

[wolfv]

This is implemented in #249

Referencing some conda-build issues on this: conda/conda-build#3308 (comment)

conda/conda-build#4142