We have strings containing hardcoded ".dylib" extensions in homebrew-core.
To be able to bring linuxbrew-core and homebrew-core closer together,
I am introducing a new generic attribute that can be used in formulae.
We're not actually using this anywhere and it makes the code
more complicated.
Relatedly, this PR fixes the issue where
`uses_from_macos "python@2" => :build` was valid on macOS but not
on Linux.