scalafix-core: implicit helpers not available with -Xsource:3 on 2.13.13

[Brice Jaglin]

Exposed by https://github.com/bjaglin/scalafix-rules/actions/runs/8108465163/job/22161731871

[error] /home/runner/work/scalafix-rules/scalafix-rules/rules/src/main/scala/fix/RemoveSamePackageImport.scala:12:22: Implicit method XtensionCollectionLikeUI was found in a package prefix of the required type, which is not part of the implicit scope in Scala 3.
[error] For migration, add `import scala.meta.transversers.Api.XtensionCollectionLikeUI`.
[error] Scala 3 migration messages are errors under -Xsource:3. Use -Wconf / @nowarn to filter them or add -Xmigration to demote them to warnings.
[error] Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=scala3-migration, site=fix.RemoveSamePackageImport.fix.pkgOpt
[error]     val pkgOpt = doc.tree.collect { case pkg: Pkg =>

We should investigate if there is a way to change the implicit structure to make that transparent.