Fix classpath issues with the dotty-sbt-bridge build

When compiling and running the tests for the bridge, we should have the
bootstrapped library on the classpath since this is what people will
actually use in the end. This is tricky to do since the bridge itself
cannot be compiled with anything bootstrapped on the classpath since the
bootstrapped projects need the bridge to be compiled !

We sidestep this issue by using two separate projects for the bridge and
its tests.

build.sbt

@@ -8,6 +8,7 @@ val `dotty-compiler-bootstrapped` = Build.`dotty-compiler-bootstrapped`
 val `dotty-library` = Build.`dotty-library`
 val `dotty-library-bootstrapped` = Build.`dotty-library-bootstrapped`
 val `dotty-sbt-bridge` = Build.`dotty-sbt-bridge`
+val `dotty-sbt-bridge-tests` = Build.`dotty-sbt-bridge-tests`
 val `dotty-language-server` = Build.`dotty-language-server`
 val `dotty-bench` = Build.`dotty-bench`
 val `dotty-bench-bootstrapped` = Build.`dotty-bench-bootstrapped`

project/Build.scala

@@ -680,19 +680,39 @@ object Build {
     case Bootstrapped => `dotty-library-bootstrapped`
   }
 
-  lazy val `dotty-sbt-bridge` = project.in(file("sbt-bridge")).
-    dependsOn(dottyCompiler(NonBootstrapped) % Provided).
+  lazy val `dotty-sbt-bridge` = project.in(file("sbt-bridge/src")).
+    // We cannot depend on any bootstrapped project to compile the bridge, since the
+    // bridge is needed to compile these projects.
     dependsOn(dottyDoc(NonBootstrapped) % Provided).
     settings(commonJavaSettings).
     settings(
       description := "sbt compiler bridge for Dotty",
-      libraryDependencies ++= Seq(
-        Dependencies.`compiler-interface` % Provided,
-        (Dependencies.`zinc-api-info` % Test).withDottyCompat(scalaVersion.value)
-      ),
+
+      sources in Test := Seq(),
+      scalaSource in Compile := baseDirectory.value,
+      javaSource  in Compile := baseDirectory.value,
+
+      // Referring to the other project using a string avoids an infinite loop
+      // when sbt reads the settings.
+      test in Test := (test in (LocalProject("dotty-sbt-bridge-tests"), Test)).value,
+
+      libraryDependencies += Dependencies.`compiler-interface` % Provided
+    )
+
+  // We use a separate project for the bridge tests since they can only be run
+  // with the bootstrapped library on the classpath.
+  lazy val `dotty-sbt-bridge-tests` = project.in(file("sbt-bridge/test")).
+    dependsOn(dottyCompiler(Bootstrapped) % Test).
+    settings(commonBootstrappedSettings).
+    settings(
+      sources in Compile := Seq(),
+      scalaSource in Test := baseDirectory.value,
+      javaSource  in Test := baseDirectory.value,
 
       fork in Test := true,
-      parallelExecution in Test := false
+      parallelExecution in Test := false,
+
+      libraryDependencies += (Dependencies.`zinc-api-info` % Test).withDottyCompat(scalaVersion.value)
     )
 
   lazy val `dotty-language-server` = project.in(file("language-server")).