[buildifier]: add lint warning for disallowed load

[alexeagle]

Many ruleset users wrap the upstream rules with a macro that sets defaults, enforces policies, or creates an additional rule.

Gazelle can generate rules with a load statement that ensures the macro is used rather than directly loading the external rule, using the map_kind directive.

However, these users still need a way to prevent their developers accidentally loading symbols from a starlark module they forbid.

For example I'd want to configure buildifier to detect

load("@rules_python//python:defs.bzl", "py_library")

and print the suggestion of using a macro instead like

load("//tools/bazel/python:defs.bzl", "py_library")

[github-actions]

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[jbedard]

This is still a request from multiple people and orgs.

PR pending and waiting for review: #1318