feat!: add remote package source to lockfile

[mrcjkb]

This PR introduces a RemoteSource type, which contains information about what type of source (e.g. a Luarocks server) and the URL a LocalPackage was pulled from.

The new field has to be an Option, because it's possible to run rocks build on a local project.

[mrcjkb]

I'm stuck on the fact that the LocalPackage is constructed in the build function, but the RemoteSource struct is pub(crate).
I don't want to make it pub, so I'll start work on #293.

[mrcjkb]

Stacked this on all the builder pattern refactors.

This ended up getting a lot bigger than I originally anticipated (sorry for that 😞).

Here's a summary of the changes:

• Changed Config::server() and ::extra_servers() to return Urls instead of Strings.
• Introduced a RemotePackageSource type, which provides information about where a remote package was fetched from.
  Currently, there are two variants:
  • LuarocksServer - with a URL
  • RockspecContent - for packages that were built from a rockspec on a local filesystem
• We serialize the RemotePackageSource and store it in the lockfile [this is a breaking change!].
  • Question: Do we want to base64 encode the RockspecContent, to make it more compact?
    My gut feeling says no, because it wouldn't be good for git diffs.
• The introduction of RemotePackageSource and its presence as a field in LocalPackage has resulted in a clippy large_enum_variant warning for RockMatches, which is why I have modified it to hold LocalPackageIds instead.
• Because RemotePackageSource is private, the LocalPackage constructor is now also private, so a bunch of public APIs that used to take a LocalPackage now take a LocalPackageId and look up the package from the Tree (for example, Remove).