Require data on cc_sysroot?

[keith]

With cc_sysroot it's valid to provide just something like this:

cc_sysroot(
    name = "sysroot",
    sysroot = "@sdk//:dir",
)

Without actually propagating the files to the actions that rely on this. As far as I can tell the expected use of this is actually do provide the data alongside this:

cc_sysroot(
    name = "sysroot",
    data = ["@sdk//:all_files"],
    sysroot = "@sdk//:dir",
)

Which makes me wonder if this should be required on the cc_sysroot macro to make it more clear what attribute is expected for this?