This PR converts complete from a separate crate to a module, and makes it available via pub mod complete. This way, callers can use e.g. uutils_args::complete::Value without having to declare an additional dependency on the (former) uutils_args_complete crate.

Background: Using a proc-macro like #[derive(Arguments)] injects code that is evaluated in the context of the destination crate. In some cases, the proc-macros in derive inject a direct access to something in the (former) crate uutils_args_complete. This doesn't work if the destination crate doesn't add it as an explicit dependency, leading to compilation errors:

error[E0433]: failed to resolve: could not find `uutils_args_complete` in the list of imported crates
  --> src/uu/du/src/du.rs:81:10
   |
81 | #[derive(Arguments)]
   |          ^^^^^^^^^ could not find `uutils_args_complete` in the list of imported crates
   |
   = note: this error originates in the derive macro `Arguments` (in Nightly builds, run with -Z macro-backtrace for more info)

There are multiple ways to resolve this issue:

• Option 1: Shrug, and tell all users that they must always add a dependency to uutils-args and also uutils-args-complete, leading to weird duplication.
• Option 2: Make complete somehow available through uutils-args itself, e.g. at the path uutils_args::complete::*, and use that path when injecting code in our derive macro.

There are two ways to make it available:

• Option 2a: pub extern crate uutils_args_complete as complete; or some abomination like that.
• Option 2b: Stop trying to keep complete a separate crate, and merge it as a module. This gets rid of some redundant metadata, and simplifies the structure, in my eyes.

This PR uses Option 2b. In fact, since #131 makes complete a non-optional dependency anyway, I don't see a point in keeping it a separate crate.

Most changes are very boring: s/::uutils_args_complete/::uutils_args::complete/g and within the (former) crate complete, the string crate:: becomes crate::complete::. The rest is just formatting. That's it.