Add support for epoch-versioned src dirs

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/build.sbt

@@ -0,0 +1,5 @@
+import sbtcrossproject.{crossProject, CrossType}
+
+lazy val foo = crossProject(JVMPlatform).settings(
+  crossScalaVersions := Seq("2.12.14", "2.13.6", "3.0.0")
+)

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/foo/shared/src/main/scala-2.12/BinaryBar.scala

@@ -0,0 +1,3 @@
+object BinaryBar {
+  val binary = "2.13"
+}

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/foo/shared/src/main/scala-2.13/BinaryBar.scala

@@ -0,0 +1,3 @@
+object BinaryBar {
+  val binary = "2.12"
+}

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/foo/shared/src/main/scala-2/EpochBar.scala

@@ -0,0 +1,3 @@
+object EpochBar {
+  val epoch = 2
+}

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/foo/shared/src/main/scala-3/BinaryBar.scala

@@ -0,0 +1,3 @@
+object BinaryBar {
+  val binary = "3"
+}

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/foo/shared/src/main/scala-3/EpochBar.scala

@@ -0,0 +1,3 @@
+object EpochBar {
+  val epoch = 3
+}

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/foo/shared/src/main/scala/Foo.scala

@@ -0,0 +1,6 @@
+object Foo {
+  def main(args: Array[String]): Unit = {
+    println(EpochBar.epoch)
+    println(BinaryBar.binary)
+  }
+}

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/project/build.properties

@@ -0,0 +1 @@
+sbt.version=1.5.4

sbt-crossproject-test/src/sbt-test/new-api/shared-cross-sources/test

@@ -0,0 +1 @@
+> +fooJVM/run

sbt-crossproject/src/main/scala/sbtcrossproject/CrossProject.scala

@@ -211,10 +211,19 @@ object CrossProject {
                            cross: Boolean): Seq[File] = {
         sharedSrcDir match {
           case Some(dir) =>
-            if (cross)
-              Seq(dir.getParentFile / s"${dir.name}-$scalaBinaryVersion", dir)
-            else
+            if (cross) {
+              val scalaEpochVersion = scalaBinaryVersion.takeWhile(_ != '.')
+              Seq(
+                Some(dir.getParentFile / s"${dir.name}-$scalaBinaryVersion"),
+                if (scalaEpochVersion != scalaBinaryVersion)
+                  Some(dir.getParentFile / s"${dir.name}-$scalaEpochVersion")
+                else
+                  None,
+                Some(dir)
+              ).flatten
+            } else {
               Seq(dir)
+            }
           case None => Seq()
         }
       }