- `fromlinecolumn` is added to be almost the reverse of `line`.
  This is needed in the future to get from `builtins.unsafeGetAttrPos`
  locations to the index for rnix
- Previously the semantics for newline indices were not really defined,
  now they are, and make more sense.
- Now there's a unit test for these functions

This makes the callPackage detection stronger by syntactically detecting
whether an attribute is using callPackage

See the added test case for why this is needed.

Makes the reference check use the nixFileCache instead of separately
parsing and resolving paths

With `cargo fmt`

Explicitly didn't do that for previous commits in order to keep the diff
more easily reviewable

NixOS#285089 (review)

Many thanks, Philip Taron!

There was too much indentation!