fix: wrap generation test labels with Label()

[pedrobarco]

What type of PR is this?

Bug fix

What package or component does this PR mostly affect?

internal/generationtest

What does this PR do? Why is it needed?

Using an hardcoded label in generationtest.bzl would prevent users that are using the gazelle_generation_test macro from using it without explicitly set repo_name = io_bazel_rules_go in MODULE.bazel.

At the moment, running without repo_name:

bazel_dep(name = "rules_go", version = "0.46.0")

$ bazel test //...
ERROR: no such package '@@[unknown repo 'io_bazel_rules_go' requested from @@]//go/tools/bazel': The repository '
@@[unknown repo 'io_bazel_rules_go' requested from @@]' could not be resolved: No repository visible as '@io_baze
l_rules_go' from main repository
ERROR: /home/barco/repos/pedrobarco/gazelle_oci/testdata/BUILD.bazel:4:28: no such package '@@[unknown repo 'io_b
azel_rules_go' requested from @@]//go/tools/bazel': The repository '@@[unknown repo 'io_bazel_rules_go' requested
 from @@]' could not be resolved: No repository visible as '@io_bazel_rules_go' from main repository and referenc
ed by '//testdata:oci_go_image'

And running with repo_name:

bazel_dep(name = "rules_go", version = "0.46.0", repo_name = "io_bazel_rules_go")

$ bazel test //...
INFO: Analyzed 5 targets (126 packages loaded, 12013 targets configured).
INFO: Found 4 targets and 1 test target...
INFO: Elapsed time: 0.907s, Critical Path: 0.08s
INFO: 7 processes: 6 internal, 1 linux-sandbox.
INFO: Build completed successfully, 7 total actions
//testdata:oci_go_image                                                  PASSED in 0.0s

With this fix, bzlmod users can drop the repo_name and still be able to use this macro.

bazel_dep(name = "rules_go", version = "0.46.0")
bazel_dep(name = "gazelle", version = "0.35.0")
local_path_override(
    module_name = "gazelle",
    path = "../../bazelbuild/bazel-gazelle",
)

$ bazel test //...
INFO: Analyzed 5 targets (126 packages loaded, 12013 targets configured).
INFO: Found 4 targets and 1 test target...
INFO: Elapsed time: 0.907s, Critical Path: 0.08s
INFO: 7 processes: 6 internal, 1 linux-sandbox.
INFO: Build completed successfully, 7 total actions
//testdata:oci_go_image                                                  PASSED in 0.0s

Which issues(s) does this PR fix?

There's no issue right now, but I've created a slack thread to address this issue