Merge pull request #242695 from tweag/lib.path.subpath.components

</details>

### Prefer returning subpaths over components

[subpath-preference]: #prefer-returning-subpaths-over-components

Observing: Functions could return subpaths or lists of path component strings.

Considering: Subpaths are used as inputs for some functions. Using them for outputs, too, makes the library more consistent and composable.

Decision: Subpaths should be preferred over list of path component strings.

<details>

<summary>Arguments</summary>

- (+) It is consistent with functions accepting subpaths, making the library more composable

- (-) It is less efficient when the components are needed, because after creating the normalised subpath string, it will have to be parsed into components again

  - (+) If necessary, we can still make it faster by adding builtins to Nix

  - (+) Alternatively if necessary, versions of these functions that return components could later still be introduced.

- (+) It makes the path library simpler because there's only two types (paths and subpaths). Only `lib.path.subpath.components` can be used to get a list of components.

  And once we have a list of component strings, `lib.lists` and `lib.strings` can be used to operate on them.

  For completeness, `lib.path.subpath.join` allows converting the list of components back to a subpath.

</details>

## Other implementations and references

- [Rust](https://doc.rust-lang.org/std/path/struct.Path.html)

              ${subpathInvalidReason path}''

      ) 0 subpaths;

  /*

  Split [a subpath](#function-library-lib.path.subpath.isValid) into its path component strings.

  Throw an error if the subpath isn't valid.

  Note that the returned path components are also valid subpath strings, though they are intentionally not [normalised](#function-library-lib.path.subpath.normalise).

  Laws:

  - Splitting a subpath into components and [joining](#function-library-lib.path.subpath.join) the components gives the same subpath but [normalised](#function-library-lib.path.subpath.normalise):

        subpath.join (subpath.components s) == subpath.normalise s

  Type:

    subpath.components :: String -> [ String ]

  Example:

    subpath.components "."

    => [ ]

    subpath.components "./foo//bar/./baz/"

    => [ "foo" "bar" "baz" ]

    subpath.components "/foo"

    => <error>

  */

  subpath.components =

    subpath:

    assert assertMsg (isValid subpath) ''

      lib.path.subpath.components: Argument is not a valid subpath string:

          ${subpathInvalidReason subpath}'';

    splitRelPath subpath;

  /* Normalise a subpath. Throw an error if the subpath isn't valid, see

  `lib.path.subpath.isValid`

      expr = (builtins.tryEval (subpath.normalise "..")).success;

      expected = false;

    };

    testSubpathComponentsExample1 = {

      expr = subpath.components ".";

      expected = [ ];

    };

    testSubpathComponentsExample2 = {

      expr = subpath.components "./foo//bar/./baz/";

      expected = [ "foo" "bar" "baz" ];

    };

    testSubpathComponentsExample3 = {

      expr = (builtins.tryEval (subpath.components "/foo")).success;

      expected = false;

    };

  };

in

  if cases == [] then "Unit tests successful"