Greetings Guix,

I'm working on creating a build system for Janet [0] modules. This is my 
first time working with build systems, and while I have a much better 
understanding than when I started, I still don't understand it fully. I've 
attached my work in progress patches. The first five set the stage by 
updating and improving our Janet package. Patch six adds the build system, 
and patch seven adds an example package, janet-posix-spawn.

[0] https://janet-lang.org

Trying to build janet-posix-spawn results in:

```
jackhill@alperton ~/repos/guix [env]$ ./pre-inst-env guix build janet-posix-spawn
The following derivations will be built:
    /gnu/store/y1xpmfaqswcmiwzpc1bgak18rbw2k4ad-janet-posix-spawn-0.0.1.drv
    /gnu/store/w52ndznq3v42xxv4lfrpdlmwgn8r0f2x-module-import-compiled.drv
building /gnu/store/w52ndznq3v42xxv4lfrpdlmwgn8r0f2x-module-import-compiled.drv...
Backtrace:
In system/base/compile.scm:
      43:4 19 (call-with-output-file/atomic _ _ _)
In ice-9/boot-9.scm:
   1736:10 18 (with-exception-handler _ _ #:unwind? _ # _)
In system/base/compile.scm:
     59:11 17 (_)
    155:11 16 (_ #<closed: file 7ffff69868c0>)
    235:18 15 (read-and-compile #<input: guix/build/janet-build-syst?> ?)
    183:32 14 (compile-fold (#<procedure compile-tree-il (x e opts)>) ?)
In ice-9/boot-9.scm:
    2806:4 13 (save-module-excursion _)
In language/scheme/compile-tree-il.scm:
     31:15 12 (_)
In ice-9/psyntax.scm:
   1241:36 11 (expand-top-sequence _ _ _ #f _ _ _)
   1233:19 10 (parse _ (("placeholder" placeholder)) ((top) #(# # ?)) ?)
    285:10  9 (parse _ (("placeholder" placeholder)) (()) _ c&e (# #) #)
In ice-9/eval.scm:
    293:34  8 (_ #<module (#{ g32}#) 7ffff697ac80>)
In ice-9/boot-9.scm:
    3380:4  7 (define-module* _ #:filename _ #:pure _ #:version _ # _ ?)
   2565:24  6 (call-with-deferred-observers _)
   3393:24  5 (_)
    222:29  4 (map1 (((guix build gnu-build-system) #:prefix gnu:) # ?))
    222:29  3 (map1 (((guix build utils)) ((guix utils))))
    222:17  2 (map1 (((guix utils))))
    3300:6  1 (resolve-interface (guix utils) #:select _ #:hide _ # _ ?)
   1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
no code for module (guix utils)
```

What have I done wrong?

More generally, I expect there are many ways I could improve the build 
system. What are they?

Thanks!
Jack