Add a //file target to external jar repositories

Fixes #990.

It's sometimes useful to be able to access the file pulled down
with maven_jar; for example, scala macro jars don't work with ijar,
so it's necessary to pull them into the compile-time classpath
unharmed.

--
Change-Id: I1d9e1cb30ea6fa56c18cbfcc68ee9b203524dfd4
Reviewed-on: https://bazel-review.googlesource.com/#/c/3550
MOS_MIGRATED_REVID=121379551

src/main/java/com/google/devtools/build/lib/bazel/repository/JarDecompressor.java

@@ -93,6 +93,12 @@ protected String createBuildFile(String baseName) {
             "    name = 'jar',",
             "    jars = ['" + baseName + "'],",
             "    visibility = ['//visibility:public']",
+            ")",
+            "",
+            "filegroup(",
+            "    name = 'file',",
+            "    srcs = ['" + baseName + "'],",
+            "    visibility = ['//visibility:public']",
             ")");
   }
 }

src/test/java/com/google/devtools/build/lib/rules/repository/JarDecompressorTest.java

@@ -49,6 +49,16 @@ public void setUpFs() throws Exception {
         .setArchivePath(jar);
   }
 
+  @Test
+  public void testTargets() throws Exception {
+    Path outputDir = DecompressorValue.decompress(descriptorBuilder.build());
+    assertThat(outputDir.exists()).isTrue();
+    String buildContent =
+        new String(FileSystemUtils.readContentAsLatin1(outputDir.getRelative("jar/BUILD")));
+    assertThat(buildContent).contains("java_import");
+    assertThat(buildContent).contains("filegroup");
+  }
+
   @Test
   public void testWorkspaceGen() throws Exception {
     Path outputDir = DecompressorValue.decompress(descriptorBuilder.build());