While unifying most of the lib function calls I accidentially changed
the filterSource functions as well. Since there were no tests I ended
up forgetting about this case (even thought I ran into it…).
We can just use `lib` instead of `builtins` in all cases but the
`hashString` case. Also changed a few lines to make use of some optional
helpers from lib.
buildRustCrate has a handy `include` helper, that only imports those whitelisted
files and folders to the store.
However, the function's matching logic is broken and includes all files,
regardless of whether or not they're whitelisted, as long as the whitelist
contains at least one name (regardless of whether that name exists). This is
because it doesn't take into account that
`lib.strings.removePrefix "foo" "bar" == "bar"` (that is, paths that don't match
the prefix are passed straight through).
