Fix BuildInfo static Java file generator (#3074)

Fix #3073

Pull request: #3074

Author: lefou

contrib/buildinfo/src/mill/contrib/buildinfo/BuildInfo.scala

@@ -139,8 +139,8 @@ object BuildInfo {
          |public class $buildInfoObjectName {
          |  $bindingsCode
          |
-         |  public static java.util.Map<String, String> toMap() {
-         |    Map<String, String> map = new HashMap<String, String>();
+         |  public static java.util.Map<java.lang.String, java.lang.String> toMap() {
+         |    java.util.Map<java.lang.String, java.lang.String> map = new java.util.HashMap<java.lang.String, java.lang.String>();
          |    $mapEntries
          |    return map;
          |  }

contrib/buildinfo/test/src/mill/contrib/buildinfo/BuildInfoTests.scala

@@ -207,6 +207,19 @@ object BuildInfoTests extends TestSuite {
       )
     }
 
+    "java-static" - workspaceTest(BuildInfoJavaStatic, "java") { eval =>
+      val runResult = eval.outPath / "hello-mill"
+      val generatedSrc = eval.outPath / "buildInfoSources.dest" / "foo" / "BuildInfo.java"
+      val Right((result, evalCount)) =
+        eval.apply(BuildInfoJavaStatic.run(T.task(Args(runResult.toString))))
+
+      assert(
+        os.exists(runResult),
+        os.exists(generatedSrc),
+        os.read(runResult) == "not-provided-for-java-modules"
+      )
+    }
+
     "generatedSources must be a folder" - workspaceTest(BuildInfoPlain, "scala") { eval =>
       val buildInfoGeneratedSourcesFolder = eval.outPath / "buildInfoSources.dest"
       val Right((result, evalCount)) = eval.apply(BuildInfoPlain.generatedSources)