Support optional dependencies, i.e.
programs.foo.packages = foo \ bar [optional]
There should be a variable if the program needs to know if bar is available, i.e.
packages.bar.exists = false
Support optional dependencies, i.e.
programs. foo.packages = foo \
bar [optional]
There should be a variable if the program needs to know if bar is available, i.e.
packages.bar.exists = false