feat: platforms-aware package arch aliases

[jjmaestro]

[!NOTE]
Stacked on top of #97

feat: platforms-aware package arch aliases

Package repos can now be used directly without the /<ARCH> bit.

The "root-level" BUILD in the package repo now has platforms-aware aliases that use select to pick the right arch for the :data, :control, etc., targets.

Now, you can do:

load("@rules_oci//oci:defs.bzl", "oci_image")

oci_image(
    name = "foo",
    base = "@distroless_base_nossl_debian12",
    tars = [
        "@debian12//coreutils",
        "@debian12//bash",
    ],
)

when before you would have needed something like:

load("@rules_oci//oci:defs.bzl", "oci_image")

CPU_ARCH = [
    ("arm64", "arm64"),
    ("x86_64", "amd64"),
]

PACKAGES = [
    "@debian12//coreutils",
    "@debian12//bash",
]

PACKAGES_ARCH = select({
    "@platforms//cpu:%s" % cpu: [
        "%s/%s" % (package, arch)
        for package in PACKAGES
    ]
    for cpu, arch in CPU_ARCH
})

oci_image(
    name = "foo",
    base = "@distroless_base_nossl_debian12",
    tars = PACKAGES_ARCH,
)

To force a specific architecture the "arch packages" can still be used just like before with @repo//package/arch or via constraints and platforms (--platforms=...).

chore: remove select() from the examples

This is a separate diff so that we can test the previous diff with the changes plus the examples with select in place and make sure everything works as-is with the new package aliases.