Prepare to split up std.format into submodules.

[berni44]

Now, that no more conflicting PRs are out there I wanted to reopen #7785, but probably got the order of steps wrong. Github now declines me doing so...

The idea is, to make very small steps here. One of the main problem of splitting modules is, that ideally no other PR should be merged during the process, because it's easy to overlook some of these changes while rebasing a splitting PR. So this PR just moves everything to std/format/package.d (with only a single slight change, that was needed).

See also PR #7765 for the main idea behind this.

Concerning the timing: In my opinion we can already do some preparatory steps before 2.096.0 is out: Moving to package.d (this PR) as well as extracting the internals. For me this would be convenient, because I could continue working on formatting floats earlier. But if you think, we should wait until March, I'll be happy to wait.

[dlang-bot]

Thanks for your pull request and interest in making D better, @berni44! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7796"

[PetarKirov]

Nice work! This is indeed small enough but useful step in the right direction.