**What type of PR is this?**

Other (slight improvement)

**What package or component does this PR mostly affect?**

cmd/gazelle

**What does this PR do? Why is it needed?**

It changes formatting of `@foo//:foo` to `@foo` in `Label.String()`.

**Which issues(s) does this PR fix?**

Fixes #2340.

**Other notes for review**

Tested with a local repo with `gazelle_cc` and it did rename e.g.
`@boost.geometry//:boost.geometry` to `@boost.geometry`.

(Disclaimer: I didn't test this PR directly, but directly applied the
change to `external/gazelle+/label/label.go` because of an issue with Go
when I tried the main `gazelle` branch with this commit).

Signed-off-by: Grégoire Geis <gregoire@seoulrobotics.org>