Support moving sources in srcjars in soong_zip

Add a -srcjar argument to soong_zip that causes it to read the
package statement of each .java file and use that to place the
source file at a path that matches the package.

Test: jar_test.go, zip_test.go
Change-Id: I36017e42445ba3b0a82a10a8d81e8ac0cca096f2

jar/Android.bp

@@ -18,8 +18,10 @@ bootstrap_go_package {
     srcs: [
         "jar.go",
     ],
+    testSrcs: [
+        "jar_test.go",
+    ],
     deps: [
         "android-archive-zip",
     ],
 }
-