Initialized from nixpkgs-architecture/simple-package-paths@01948e0

In addition to some more minor changes and incorporating feedback, the
major changes are:
- Restructure the RFC into two separate parts, one to introduce
  the convention and one to migrate packages to it when possible
- Remove the restriction that files inside a unit directory can only be
  referenced by the corresponding `pkgs.${name}`. It feels very
  unnatural to have this restriction and it's hard to reason about it.
  Files inside a unit directory still can't reference anything _outside_
  the unit directory, which is very similar to Nix's concept of allowed-uris,
  which may be used to implement this check in the future.
- Remove the special case of allowing custom arguments. By not having
  this one exception, users viewing a unit directory can be sure that
  there's no hidden semantics anywhere (overriding arguments) and that the
  functions arguments correspond directly to attributes in `pkgs`, no
  exceptions that would require looking at `all-packages.nix`.
  And it would be weird just to allow this one exception of
  `callPackage` with custom arguments, when there's a lot of other
  similarly small exceptions we could make, like allowing
  `python3Packages.callPackage`.
- Remove the requirement that new packages must use this standard.
  Especially with the above exception removed, this standard is now more
  strict and less packages satisfy it by default.
  A scenario could be that a user adds a new package, initially not
  needing custom arguments, so CI requires it to be in `pkgs/unit`, but
  then a custom argument is needed, so it must be moved out of there and
  added to `all-packages.nix`. But then the custom argument can be removed,
  so it _must_ be in `pkgs/unit` again. This sucks.
  So let's keep `all-packages.nix` unrestricted, so a package won't have
  to be moved back and forth like this.

Co-Authored-By: Robert Hensing <[email protected]>

And some very minor changes

Remove the barely used term "base directory"

Co-authored-by: Robert Hensing <[email protected]>