[wip] layer_from_pkg

[peakschris]

Closes #188.

Adds layer_from_pkg to //img:layer.bzl, which converts rules_pkg packaging manifests into OCI image layers. It accepts PackageFilesInfo, PackageFilegroupInfo, PackageSymlinkInfo, and PackageDirsInfo from rules_pkg_providers — a lightweight BCR module whose only dependency is rules_license, so rules_pkg itself is not pulled in as a transitive dependency of rules_img.

layer_from_pkg(
    name = "app_layer",
    srcs = [":app_pkg"],  # pkg_filegroup, pkg_files, pkg_mklink, pkg_mkdirs
)

Waiting on rules_pkg

rules_pkg 1.2.0 has not yet migrated to emit rules_pkg_providers providers — it still defines them inline. Until a new rules_pkg release adopts rules_pkg_providers, targets built with pkg_files / pkg_filegroup cannot be passed to layer_from_pkg directly.

For now, the e2e tests work around this with fake_pkg_* fixtures that mirror the rules_pkg API but emit providers from rules_pkg_providers directly. These fixtures can be replaced with real rules_pkg targets once that migration lands.