Use dash instead of underscore in attribute names

Nix supports dashes now, and using dashes allows attributes to be the
same as package names.
Use dashes between version components in attrnames

So that build input attributes look like "foo-0-15" instead of
"foo-0.15". Having '.' in attributes is a bit cumbersome in nix, and the
manual suggests to use dashes everywhere.

(Although I'm uncertain often it's useful to to have versioned
attributes in build inputs.)