Feature-request: Better `use_extension` API for dev dependencies

[luispadron]

Description of the feature request:

In rules_swift_package_manager we have a module extension which generates repositories for a Swift package representation, it also generates two other repositories for use as "development" dependencies (e.g. swift_deps_info, and swift_package). These repositories are not meant to be exposed outside of the root repo that the extension is used in.

The user-experience issue:

We could define these repositories as root_module_direct_dev_deps but doing so requires the extension to be in a use_extension with dev_dependency = True which means we need to split our extension into development and non-development. This complicates implementing extensions but it also complicates using them if the extensions shares attributes across dev/non-dev, you end up duplicating or creating a constant to reuse.

This ends up looking something like:

# Declare public deps
swift_deps = use_extension(
    "@rules_swift_package_manager//:extensions.bzl",
    "swift_deps",
)
swift_deps.from_package(
    swift = "//:Package.swift",
    ...
)
use_repo(
    swift_deps,
    ...
    "swiftpkg_swiftformat",
)

# Declare dev deps
swift_dev_deps = use_extension(
    "@rules_swift_package_manager//:extensions.bzl",
    "swift_dev_deps",
    dev_dependency = True,
)
swift_dev_deps.from_package(
    swift = "//:Package.swift",
    ...
)
use_repo(swift_dev_deps, "swift_package")

Alternative

# Declare deps
swift_deps = use_extension(
    "@rules_swift_package_manager//:extensions.bzl",
    "swift_deps",
)
swift_deps.from_package(
    swift = "//:Package.swift",
    ...
)
use_repo(
    swift_deps,
    "swiftpkg_swiftformat",
    dev_dependencies = [
       "swift_package"
    ],
)

Ideally, extension authors could just define dev dependencies without users needing to use dev_dependency = True but I'm not sure how to functionally make that work. Just think the end user experience here is less than ideal for more complicated Bazel modules.

[fmeum]

dev_dependency is a property of the extension usage, so a user would be able to add both a regular and a dev use_extension for the same module extension. You wouldn't have to repeat the tags. Would that already improve the situation?

It's possible that bazel mod tidy could do a better job in that case, up to possibly even adding the dev usage.