build.zig dependencies: support url mirrors

[andrewrk]

Extracted from #14265.

.{
    .name = "libffmpeg",
    .version = "5.1.2",
    .dependencies = .{
        .libz = .{
            .url = .{
                "https://github.com/andrewrk/libz/archive/f0e53cc2391741034b144a2c2076ed8a9937b29b.tar.gz",
                "https://andrewkelley.me/temp/backup (2).tar.gz",
            },
            .hash = "sha256=c9b30cffc40999d2c078ff350cbcee642970a224fe123c756d0892f876cf1aae",
        },
    },
}

Note that the hash is the same regardless of which mirror URL is used.

URL mirrors are tried in succession; if one mirror fails to deliver a directory of files with a matching hash, then the next URL is tried, and so on until all the mirrors have been tried.

[motiejus]

Some implementation details of a different package manager that does it as described: https://bazel.build/rules/lib/repo/http#http_archive-urls ; note urls is in the same scope as sha256, exactly like in this proposal.

Bazel maintains these files in ~/.cache/bazel/_bazel_<username>/cache/repos/v1/content_addressable/sha256. For example, https://<...>/java_tools_linux-v11.7.1.zip, whose sha256 is f78077f0c043... is stored like this:

$ cat ~/.cache/bazel/_bazel_motiejus/cache/repos/v1/content_addressable/sha256/f78077f0c043d0d13c82de0ee4a99753e66bb18ec46e3601fa2a10e7f26798a8/id-d5b73c8c527f33efb260613ab75629768c6244457266cd6a1009a996883b8956; echo
https://mirror.bazel.build/bazel_java_tools/releases/java/v11.7.1/java_tools_linux-v11.7.1.zip https://github.com/bazelbuild/java_tools/releases/download/java_v11.7.1/java_tools_linux-v11.7.1.zip

$ sha256sum ~/.cache/bazel/_bazel_motiejus/cache/repos/v1/content_addressable/sha256/f78077f0c043d0d13c82de0ee4a99753e66bb18ec46e3601fa2a10e7f26798a8/file 
f78077f0c043d0d13c82de0ee4a99753e66bb18ec46e3601fa2a10e7f26798a8  /home/motiejus/.cache/bazel/_bazel_motiejus/cache/repos/v1/content_addressable/sha256/f78077f0c043d0d13c82de0ee4a99753e66bb18ec46e3601fa2a10e7f26798a8/file

And before fetching the file it looks at the cache directory.