When declaring a dependency pandas[excel,html,plot], the lockfile shows:

dependencies = [
    { name = "pandas" },
    { name = "pandas", extra = "excel" },
    { name = "pandas", extra = "html" },
    { name = "pandas", extra = "plot" },
]

We should collapse those entries:

dependencies = [
    { name = "pandas", extra = ["excel", "html", "plot"] },
]

This makes more sense to the user (we don't install pandas four times, we install it once and then those extra packages, that's our internal abstraction into virtual packages leaking) and makes the lockfile more concise.