Publish proguarded module built with scala 2.12 too (#78)

Published as `io.get-coursier:interface_2.12` (but doesn't depend on
scala). This module shades then proguards scala 2.12 instead of scala
2.13, which can be preferred by some users down the line (when building
GraalVM native images in particular).

Author: alexarchambault

build.sbt

@@ -22,7 +22,6 @@ lazy val finalPackageBin = taskKey[File]("")
 lazy val interface = project
   .enablePlugins(SbtProguard)
   .settings(
-    skip.in(publish) := scalaVersion.value != Settings.scala213,
     finalPackageBin := {
       import org.pantsbuild.jarjar._
       import org.pantsbuild.jarjar.util.StandaloneJarProcessor
@@ -109,7 +108,12 @@ lazy val interface = project
     ),
 
     autoScalaLibrary := false,
-    crossVersion := CrossVersion.disabled,
+    crossVersion := {
+      if (scalaVersion.value == Settings.scala213)
+        CrossVersion.disabled
+      else
+        CrossVersion.binary
+    },
 
     // filtering out non cross versioned module in 0.0.1 (published cross-versioned there, added below)
     mimaPreviousArtifacts := mimaPreviousArtifacts.value.filter(_.revision != "0.0.1"),