Figure out splitting/merging derivations

[artemist]

We currently have a large number of small derivations, and this is only increasing. e.g. zfs will require 12 more, many vary similar

We should figure out the structure of the FreeBSD directory. I think there's 3 main options:

• Keep it the way it is, many tiny derivations
• Move some grouped libraries/binaries into single derivations, possibly based on "packages" defined in the FreeBSD makefiles. Currently "bin" and "libc" basically do this
• Group several derivations into a single file, then inherit in default.nix

I think I support the 3rd option, it keeps dependencies clear but reduces the large number of files and code duplication we need, especially in things like zfs