Add support for github packages

[jmackie]

Extends the PackageSpec type to support GitHub repositories and updates the install logic.

A GitHub package dependency is specified like:

[package-set.packages.js-ref]
github = { owner = "ditto-lang", repo = "js-ref" }     # inspired by Nix's fetchFromGitHub
revision = "fb8a458da5a9394afc199974ed880bf9e99c7571"
sha256 = "a25b7f0f65ce48157ca6ff536b675c7fdd1383c6b901991257328bbd6189cf3d"

The install logic is then:

1. GET https://github.com/{github.owner}/{github.repo}/archive/{revision}.zip
2. Check that the sha256 matches
3. Store the downloaded zip archive in the cache, using the sha256 as the key (much like Nix)
4. Unpack contents of the archive into the current project's package directory

And if the entry already exists in the cache, we skip straight to 4.

[codecov]

Codecov Report

Merging #109 (e054746) into main (445761e) will decrease coverage by 0.63%.
The diff coverage is 3.75%.

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
- Coverage   85.12%   84.48%   -0.64%     
==========================================
  Files          83       83              
  Lines       14942    15062     +120     
==========================================
+ Hits        12719    12725       +6     
- Misses       2223     2337     +114     

Impacted Files	Coverage Δ
crates/ditto-cli/src/pkg.rs	2.29% <0.00%> (-0.94%)	⬇️
crates/ditto-config/src/package_set.rs	83.33% <100.00%> (+0.98%)	⬆️
crates/ditto-config/src/tests.rs	100.00% <100.00%> (ø)

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