feat: Support multiple dependency index files and document the format

[AttilaTheFun]

Per our discussion on Slack, I'd like to be able to support multiple swift_deps_index files and document the required format.

The use case is for consumers with multiple Bazel repositories, some of which do not have Swift packages. I would like to be able to supply additional indices which map module names to Bazel targets which would not otherwise be known to the Swift gazelle plugin.

Initially this could just be allowing multiple data dependencies and allowing the -swift_dependency_index flag to be passed multiple times:

gazelle(
    name = "update_build_files",
    data = [
        "@swift_deps_info//:swift_deps_index",
        "@my_swift_deps_info//:my_swift_deps_index",
    ],
    extra_args = [
        "-swift_dependency_index=$(location @swift_deps_info//:swift_deps_index)",
        "-swift_dependency_index=$(location @my_swift_deps_info//:my_swift_deps_info)",
    ],
    gazelle = ":gazelle_bin",
)

These would be overlaid on top of each other, so the last value for any given dependency would "win".f

In the future we could also possibly upstream support for user-provided resolution indices into gazelle itself, since this seems like a natural extension of the # gazelle:resolve directive.

I started a discussion around the possible gazelle integration here:
https://bazelbuild.slack.com/archives/C01HMGN77Q8/p1720377154969709