ExternalDependencyModuleIds.scala

import sbt.*
import sbt.Keys.*
import sjsonnew.BasicJsonProtocol
import sjsonnew.BasicJsonProtocol.*
import sjsonnew.Builder
import sjsonnew.JsonFormat
import sjsonnew.JsonWriter
import sjsonnew.support.scalajson.unsafe.CompactPrinter

object ExternalDependencyModuleIds extends AutoPlugin {
  override def trigger = allRequirements

  private final case class Lib(groupId: String, artifactId: String, version: String)

  private final object Lib {
    implicit val instance: JsonFormat[Lib] =
      BasicJsonProtocol.caseClass3(apply, unapply)("groupId", "artifactId", "version")

    implicit val ordering: Ordering[Lib] = Ordering.by(unapply)
  }

  override lazy val projectSettings: Seq[Def.Setting[?]] =
    Def.settings(
      TaskKey[Seq[ModuleID]]("externalDependencyModuleIds") := {
        val result = (Compile / externalDependencyClasspath).value.flatMap(_.metadata.get(moduleID.key))
        val libs = result.map { x =>
          val suffix = x.crossVersion match {
            case _: CrossVersion.Binary =>
              s"_${scalaBinaryVersion.value}"
            case _ =>
              // TODO
              ""
          }
          Lib(
            groupId = x.organization,
            artifactId = s"${x.name}$suffix",
            version = x.revision
          )
        }.sorted.map(x => s"  ${x.toJsonString}")
        IO.write(
          file(s"${name.value}.json"),
          libs.mkString("{\n", ",\n", "\n}\n")
        )
        result
      }
    )

  private implicit class JsonClassOps[A](private val self: A) extends AnyVal {
    def toJsonString(implicit format: JsonWriter[A]): String = {
      val builder = new Builder(sjsonnew.support.scalajson.unsafe.Converter.facade)
      format.write(self, builder)
      CompactPrinter(
        builder.result.getOrElse(sys.error("invalid json"))
      )
    }
  }

}

ExternalDependencyModuleIdsText.scala

import sbt.*
import sbt.Keys.*

object ExternalDependencyModuleIds extends AutoPlugin {
  override def trigger = allRequirements

  private final case class Lib(groupId: String, artifactId: String, version: String) {
    def asString: String = s"$groupId:$artifactId:$version"
  }

  private final object Lib {
    implicit val ordering: Ordering[Lib] = Ordering.by(unapply)
  }

  override lazy val projectSettings: Seq[Def.Setting[?]] =
    Def.settings(
      TaskKey[Seq[ModuleID]]("externalDependencyModuleIds") := {
        val result = (Compile / externalDependencyClasspath).value.flatMap(_.metadata.get(moduleID.key))
        val libs = result.map { x =>
          val suffix = x.crossVersion match {
            case _: CrossVersion.Binary =>
              s"_${scalaBinaryVersion.value}"
            case c if c == CrossVersion.disabled =>
              ""
            case _ =>
              sys.error(s"not supported cross version $x")
          }
          Lib(
            groupId = x.organization,
            artifactId = s"${x.name}$suffix",
            version = x.revision
          )
        }.sorted.map(_.asString)
        IO.write(
          file(s"${name.value}.txt"),
          libs.mkString("", "\n", "\n")
        )
        result
      }
    )

}