Add ability to construct a LazyPath out of any file from a dependency package

[antlilja]

Partly closes #16643

This PR implements dependencyPath, and allows for fetching dependencies without a build.zig.

This PR adds support for non zig dependencies (dependencies without a build.zig). It adds a function path to the Dependency struct which returns a LazyPath to a path inside the dependency.

It does not currently fetch these dependencies lazily, it instead just uses the current way of fetching dependencies before running the build. Doing actual lazy fetching should probably be implemented separately together with lazy dependency fetching a zig dependencies. Right now it would require duplicating a lot of work from the existing dependency fetching.

I also created a proof of concept project based on Andrews SDL which downloads a tagged release of SDL from the SDL website and builds a static library: https://github.com/antlilja/zig-sdl.

[mlugg]

IMO this should be a method on Dependency, with the LazyPath variant storing a *Dependency instead of directly storing the build root. (This makes the usage (b.dependency("foo").path("foo/bar.txt").)

[antlilja]

IMO this should be a method on Dependency, with the LazyPath variant storing a *Dependency instead of directly storing the build root. (This makes the usage (b.dependency("foo").path("foo/bar.txt").)

I tried implementing it like this instead and I agree, at least the example I provided becomes much cleaner when not having a seperate dependencyPath function.

[andrewrk]

Thanks for working on this - this is going to make a lot of people happy. I love your SDL proof of concept too. A couple requests-

[mlugg]

Oh also, wrt lazy dep fetching: this is planned to be implemented in future. I've discussed it a little with Andrew, and the basic idea is this:

• initially, we fetch no dependencies, and run the build script
• when your build script calls dependency for a dep which isn't currently downloaded, it communicates this requirement to the build runner
• the build runner fetches that package, and re-runs the build script with that dependency in place
• rinse and repeat until we have all deps we need

Obviously this recompiles the build script a bunch, so we really want to wait for the self-hosted backends to be ready for this to make those rebuilds fast.

Anyways, the point here is that optional deps won't be specific to path uses or anything, so it's fine for this code to not touch them: once we implement that it should apply to dependencies of any kind.

[antlilja]

My first implementation was a bit hacky so I've rewritten it a bit. Dependencies which don't contain a build.zig file now don't create package, they are instead just added to build_root_source. I think this makes a bit more sense and cleans up the previous implementation quite a lot (for example not requiring changes in Compilation.zig).

[ikskuh]

Dis gonna be gud!

[mlugg]

Looks good - a couple minor points.