Remove external dependency on flake-utils

[irgolic]

Hey, thanks for creating such a useful abstraction over VMs!

Would you be open to reducing the size of your dependency tree? flake-utils is only used for the eachSystem function. Inlining or replacing it cuts the project's lockfile in half (as flake-utils itself depends on another flake).

All of these commits remove the external dependency in different ways; let me know if you would like me to amend or remove any of them:

• The first commit (57e1244) inline-vendors forAllSystems, including an MIT header.
• The second commit (d5f82d0) replaces it with a simpler function that retains parity.
• The third commit (37af54c) removes the behavior of automatically including currentSystem if the flake is run with --impure. I'm guessing that most developers depending on flake-utils are unaware of this behavior – in my opinion explicitly relying on nix-systems for this behavior is a better way to achieve this goal.

While this PR aims to remove the external dependency on flake-utils in the simplest way possible, in the long term, I suggest rewriting the flake with a forAllSystems function for better readability (you won't need to merge two big attrsets, instead keeping it all top-level).

[SuperSandro2000]

First of all the git history of this is not very at all nice. This should be one commit, as otherwise using git blame becomes needlessly way harder to use and we have lots of back and forth.

Then why not use lib.foldAttrs? That would make the code probably way shorter and easier to read.

You linked https://ayats.org/blog/no-flake-utils#do-we-really-need-flake-utils that is pretty short and if we touch that part of the code, we should go with that and do it completely.

Inlining or replacing it cuts the project's lockfile in half (as flake-utils itself depends on another flake).

True, but that wouldn't matter to much in any flakes I know where microvm is used, as there are usually at least a few other flake-utils. But for people not using 20 other flakes, there is a small benefit.

[SuperSandro2000]

see above comment

[irgolic]

Thanks! Indeed, microvm-nix is currently the only flake I depend on that uses flake-utils.

I've changed this PR to consist of only one commit, implementing the pattern at https://ayats.org/blog/no-flake-utils#do-we-really-need-flake-utils. It turned out pretty clean, and removed a level of indentation from most of the file.

Let me know if there's anything else you'd like me to amend.