[plugins] Unclear how to succinctly incorporate plotjugger data

[EricCousineau-TRI]

I am trying to use plotjugger in our internal codebase (Anzu). I envisioned doing something like this in a BUILD.bazel:

anzu_ros_sh_alias(
    name = "plotjuggler",
    data = [
        # Ensures we can access the ROS 2 subscriber topic.
        "@ros2//:plotjuggler_ros_cc",
    ],
    # Provides access to custom-generated messages.
    py_interface_deps = ["//:ros_msgs_all_py"],
    target = "@ros2//:plotjuggler_plotjuggler",
)

However, to make this work, I had to do hack our our ros2/repository.bzl ( click to expand)

_ROS_PACKAGES = [
...
    "plotjuggler",
    "plotjuggler_ros",
...
]

_BUILD_EXTRA_TPL = """
# Extra content for Anzu.

cc_library(
    name = "plotjuggler_ros_cc",
    data = [
        ":plotjuggler_ros_share",
        ":plotjuggler_ros_transitive_py",
    ] + glob(
        ["${workspace_prefix}/lib/plotjuggler_ros/lib*.so*"],
        allow_empty=False,
    ),
)
"""

def _append_to_file(repo_ctx, file, content):
    orig_content = repo_ctx.read(file)
    new_content = orig_content + "\n" + content
    repo_ctx.file(file, new_content)

def _anzu_ros2_local_repository_impl(repo_ctx):
...

    base_ros2_repository(repo_ctx, workspaces_in_sandbox)

    workspace_local_path = repo_ctx.attr.workspaces[0].replace("/", "_")
    build_extra = _BUILD_EXTRA_TPL.replace(
        "${workspace_prefix}",
        workspace_local_path,
    )
    _append_to_file(repo_ctx, "BUILD.bazel", build_extra)

Questions

1. Why did I have to inject my own plotjuggler_ros_cc library? I would've thought this would be caught by scanning.
2. Why did I have to include plotjuggler_ros_transitive_py? I would've thought plotjuggler_ros_share should have covered it.