Add `projectMatrixBaseDirectory` setting

Similar to https://github.com/portable-scala/sbt-crossproject/pull/152

src/main/scala/sbt/internal/ProjectMatrix.scala

@@ -270,7 +270,8 @@ object ProjectMatrix {
             inConfig(Compile)(makeSources(nonScalaDirSuffix, svDirSuffix)),
             inConfig(Test)(makeSources(nonScalaDirSuffix, svDirSuffix)),
             projectDependencies := projectDependenciesTask.value,
-            ProjectMatrixKeys.virtualAxes := axes
+            ProjectMatrixKeys.virtualAxes := axes,
+            ProjectMatrixKeys.projectMatrixBaseDirectory := base,
           )
           .settings(self.settings)
           .configure(transforms: _*)

src/main/scala/sbtprojectmatrix/ProjectMatrixPlugin.scala

@@ -7,6 +7,7 @@ import scala.language.experimental.macros
 
 trait ProjectMatrixKeys {
     val virtualAxes = settingKey[Seq[VirtualAxis]]("Virtual axes for the project")
+    val projectMatrixBaseDirectory = settingKey[File]("Base directory of the current project matrix")
 }
 
 object ProjectMatrixKeys extends ProjectMatrixKeys

src/sbt-test/projectMatrix/jvm-sandwich/build.sbt

@@ -41,6 +41,7 @@ lazy val bazApp = (projectMatrix in file("baz-app"))
       streams.value.log.info(cp.toString)
       assert(cp.contains("baz-core_2.13-0.1.0-SNAPSHOT.jar"), s"$cp")
       assert(!cp.contains("baz-core_3.0.0-M1-0.1.0-SNAPSHOT.jar"), s"$cp")
+      assert(projectMatrixBaseDirectory.value == file("baz-app"))
     },
   )
   .jvmPlatform(scalaVersions = Seq(scala213))

src/sbt-test/projectMatrix/jvm/build.sbt

@@ -19,6 +19,7 @@ lazy val core = (projectMatrix in file("core"))
   .settings(
     check := {
       assert(moduleName.value == "core", s"moduleName is ${moduleName.value}")
+      assert(projectMatrixBaseDirectory.value == file("core"))
     },
   )
   .jvmPlatform(scalaVersions = Seq(scala213, scala212))
@@ -27,6 +28,7 @@ lazy val intf = (projectMatrix in file("intf"))
   .settings(
     check := {
       assert(moduleName.value == "intf", s"moduleName is ${moduleName.value}")
+      assert(projectMatrixBaseDirectory.value == file("intf"))
     },
   )
   .jvmPlatform(autoScalaLibrary = false)

src/sbt-test/projectMatrix/projectAxes/build.sbt

@@ -15,6 +15,7 @@ lazy val core = (projectMatrix in file("core"))
   .settings(
     check := {
        assert(platformTest.value.endsWith("-platform"))
+       assert(projectMatrixBaseDirectory.value == file("core"))
     },
   )
   .jvmPlatform(scalaVersions = Seq(scala213, scala212))