bazel_gazelle dependency needs to be moved out of // (the root package)

[jacky8hyf]

To reproduce this issue:

1. Go to commit 6ad7743
2. Do the following edits:

e2e/smoke/BUILD

load("@bazel_skylib//rules:build_test.bzl", "build_test")

alias(
    name = "empty",
    actual = "@com_myorg_rules_mylang//mylang:resolved_toolchain",
)

build_test(
    name = "smoke_test",
    targets = [
        # targets you add above
        ":empty",
    ],
)

Then:

cd e2e/smoke && bazel build //:empty

This will fail with the following error message on Bazel 8.3.1:

ERROR: error loading package '@@com_myorg_rules_mylang+//': Unable to find package for @@[unknown repo 'bazel_gazelle' requested from @@com_myorg_rules_mylang+]//:def.bzl: The repository '@@[unknown repo 'bazel_gazelle' requested from @@com_myorg_rules_mylang+]' could not be resolved: No repository visible as '@bazel_gazelle' from repository '@@com_myorg_rules_mylang+'.
ERROR: <HOME>/.cache/bazel/_bazel_elsk/176206515ed6554e22e9ad36c6477dc2/external/com_myorg_rules_mylang+/mylang/BUILD.bazel:10:15: every rule of type toolchain_type implicitly depends upon the target '@@com_myorg_rules_mylang+//:package_metadata', but this target could not be found because of: error loading package '@@com_myorg_rules_mylang+//': Unable to find package for @@[unknown repo 'bazel_gazelle' requested from @@com_myorg_rules_mylang+]//:def.bzl: The repository '@@[unknown repo 'bazel_gazelle' requested from @@com_myorg_rules_mylang+]' could not be resolved: No repository visible as '@bazel_gazelle' from repository '@@com_myorg_rules_mylang+'.
ERROR: <HOME>/.cache/bazel/_bazel_elsk/176206515ed6554e22e9ad36c6477dc2/external/com_myorg_rules_mylang+/mylang/BUILD.bazel:15:19: Analysis failed
ERROR: Analysis of target '//:empty' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.076s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

To resolve this issue, I think the root BUILD.bazel file cannot have a dependency on bazel_gazelle; it needs to be moved to another package. We can use an alias to keep backwards compatibility.