Factor out SkipDrive

[dfabulich]

skipstone depends on https://github.com/skiptools/skip/tree/main/Sources/SkipDrive in a bizarre way, by symlinking SkipDriveExternal to ../../skip/Sources/SkipDrive

Instead, there should be a skip-drive library containing the SkipDrive code; skip and skipstone should depend on it.

[marcprux]

Agreed, it's a weird setup. We needed some shared code that skipstone and SkipDrive could depend on, but it wasn't quite enough to justify an entire new repository just for this. We'd be happy for skipstone to have a normal dependency on a shared target in the skip package, but this leads to a circular dependency when skipstone is being developed locally (SwiftPM assess dependencies on a project level rather than a module level). And we couldn't do the inverse (have skip depend on skipstone) previously, because skipstone wasn't a public repository. That could theoretically change now, but I don't like the weird transitive dependencies that would appear in projects as a result (ELFKit, SwiftSyntax, Universal).

[dfabulich]

Factoring out a separate project would resolve that, right? (Is a PR welcome for this?)

[marcprux]

A separate project would resolve it, but I'm still not sure it is worth it, since it is another dependency that shows up in Xcode and Swift projects. The weird structure only really affects people doing local skipstone development work, which is a relatively uncommon scenario.

Let us think on it a bit before we go ahead with a PR.

[dfabulich]

FWIW, I filed this issue because I was trying to innocently add import SkipDriveExternal to DoctorCommand, and the whole build blew up in a really confusing way. (TSCUtility and TSCBasic provide their own Version, Thread, URL, conflicting with Foundation.)

[marcprux]

Agreed, this is super confusing. skipsone has the full swift-tools-support-core dependency, but skip itself just vendors a subset of what it needs in ToolSupport.swift. Again, this is to keep skip dependency-free and quick to build.

We should probably prefix all the types that we vendor in skip (e.g. SkipTSCVersion instead of Version, etc.) to make this clearer.

[marcprux]

Not a complete fix, but #183 should address this somewhat by including skip as a submodule and having the SkipDriveExternal link to that, rather than assuming that there is a peer skip repository checked out.