Packages that use Conda Virtual Packages break Build

[michaelweinold]

Packages that use Virtual Packages (like __unix) in their build recipes cause the build to fail with an error of the sort:

├─ pysocks 1.7.1 would require
    │     │     │     │  └─ __win  , which is missing on the system;
    │     │     │     └─ pysocks 1.7.1 would require
    │     │     │        └─ __unix  , which is missing on the system;

This was discussed in:

• Upgrade to 0.9.0 results in package is excluded by strict repo priority jupyterlite/xeus-python-kernel#148 (comment)

Related Conda documentation:

• Managing Virtual Packgages