This makes them unique throughout the package, allowing this code to be built as a single, huge module.

This makes this name unique throughout the package, allowing this code to be built as a single, huge module.

…ernal or public depending on COLLECTIONS_SINGLE_MODULE

This is a thoroughly disgusting solution, but needs must. 😞

Hopefully macros will let us eventually replace this mess.

- Sources/_CollectionsTestSupport → Tests/_CollectionsTestSupport
- Refactor Package.swift to allow switching between the default
   polymodule and the new monomodule product configurations
   (by editing the manifest file).
- Fix build errors in the monomodule variant.

Also, remove COLLECTIONS_ENABLE_UNSTABLE_MODULES as it is no longer doing anything.

This is going to ship as is, with no promises about it remaining in future versions. It is going to be a load bearing component of the package, though — just not necessary a stable one.

…& Xcode/ are unstable