Scalafix migrations are not applied to scripted sources

[bjaglin]

See scalacenter/sbt-scalafix#57:

• scalatest was bumped in the main build, and scalafix migrations were applied https://github.com/scala-steward/sbt-scalafix/blob/update/scalatest-3.1.0/src/test/scala/scalafix/internal/sbt/SbtCompletionsSuite.scala#L9
• scalatest was bumped in the scripted builds, but scalafix migrations were not applied https://github.com/scala-steward/sbt-scalafix/blob/update/scalatest-3.1.0/src/sbt-test/sbt-scalafix/basic/tests/src/test/scala/tests/CheckSuite.scala#L7

Scalafix migration:

• https://github.com/fthomas/scala-steward/blob/master/modules/core/src/main/resources/scalafix-migrations.conf#L64
• https://github.com/scalatest/autofix/blob/master/3.1.x/rules/src/main/scala/org/scalatest/autofix/v3_1_x/RewriteDeprecatedNames.scala#L13

[bjaglin]

This is quite meta, as the example repo is sbt-scalafix itself, but it could be anywhere obviously... Anyway, this most likely requires not-so-trivial changes on the sbt-scalafix side, I am investigating and will probably file an issue there as well.

[bjaglin]

Blocked by scalacenter/scalafix#1171