lib/path/README.md: Justify returning subpaths

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### 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.

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## Other implementations and references

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