Add optional dependencies to the universal_hub

[aignas]

Whilst thinking about #3594, I realized that it would be good to add dependencies if they are present, but not fail if they are not present.

API that would be good:

    deps = [
        "@pypi//must",
    ] + select({
        "@pypi//optional:is_present": ["@pypi//optional"],
        "//conditions:default": [],
    }),

[rickeylev]

This is an interesting idea. I think I like it. All this really needs is the same alias() logic for the regular targets, except the "not is present" case maps to a config_setting that is always false.

That said -- can we give some more concrete example use cases?

An alternative API I was thinking of was to move the is_present target out of the package it is checking for. e.g. @pypi_present//:is_foo_present, or something like that. I don't think it brings much, though. The hub repo is pretty lightweight, since its just a bunch of aliases.

Another option would be to push the is_present check onto the user to define their settings. Something like:

load("@pypi//:packages.bzl", "packages")

config_setting(
  name = "is_foo_present"
  flags = {} if "foo" in packages else {":always_false": "never-matches"}
)

# or maybe instead use some helpers:

load("@pypi//:packages.bzl", "package_exists_config_setting")

# macro to define the above, essentially.
package_exists_config_setting(
  name = "foo",
)

I wonder if we could generalize this to extras? e.g. is_present[extra] ?