Buildpack API inconsistency around naming/data-type of targets.distros.version field in buildpack.toml

[edmorley]

In Buildpack API 0.10, the concept of stacks was replaced by targets:

• RFC: https://github.com/buildpacks/rfcs/blob/main/text/0096-remove-stacks-mixins.md
• Spec update PR: Finalize Buildpack/0.10 #365

Part of the targets feature is the concept of distro names/versions.

For example, the spec currently gives an example buildpack.toml as:

[[targets]]
os = "<OS name>"
arch = "<architecture>"
variant = "<architecture variant>"
[[targets.distros]]
name = "<OS distribution name>"
version = "<OS distribution version>"

(from https://github.com/buildpacks/spec/blob/87dc65b9611f88349cead9676e22657349072c90/buildpack.md#buildpacktoml-toml)

And also gives a similar example here:

Tools reading `buildpack.toml` will translate any section that sets `stacks.id = "io.buildpacks.stacks.bionic"` to:

```toml
[[targets]]
os = "linux"
arch = "amd64"
[[targets.distros]]
name = "ubuntu"
version = "18.04"
```

(from https://github.com/buildpacks/spec/blob/87dc65b9611f88349cead9676e22657349072c90/buildpack.md#buildpacktoml-toml-stacks-array)

However, elsewhere in the spec, the version field has a different name, versions:

Each target in `targets`:
- MUST identify a compatible runtime environment:
   - ...
   - `distros` if provided MUST describe the OS distributions supported by the buildpack
     - For Linux-based images, `distros.name` and `distros.versions` SHOULD contain the values specified in `/etc/os-release` (`$ID` and `$VERSION_ID`), as the `os.version` field in an image config may contain combined distribution and version information
     - For Windows-based images, `distros.name` SHOULD be empty; `distros.versions` SHOULD contain the value of `os.version` in the image config (e.g., `10.0.14393.1066`)

(from https://github.com/buildpacks/spec/blob/87dc65b9611f88349cead9676e22657349072c90/buildpack.md#targets-1)

It seems the original RFC used the name versions with data-type array:
https://github.com/buildpacks/rfcs/blob/ce8991b3d739f763b8356ae8696ae582f18c4af6/text/0096-remove-stacks-mixins.md#base-image-metadata

...but that the field was renamed/changed to a string in this commit when the changes were added to the RFC:
8652ec5

And version is also what's used in the implementation:
https://github.com/buildpacks/lifecycle/blob/cd50eb8848492eac9b58c56d060d7d47ac56c693/buildpack/bp_descriptor.go#L39-L43

As such, I think the version usages in the spec are what was intended, and we should standardise everything on that.

I'll open PRs against the spec. (And I'll also backport the changes to the RFC as part of the existing PR I have open for that: buildpacks/rfcs#310)

cc @natalieparellano