Improve package loading error message on directories without BUILD files

Let's print an actionable for the user and inform that BUILD files turn directories into packages.

Fixes #9364

Closes #9385.

Change-Id: Ia19133d2bfb0e71fdd03043592f525afddff740d
PiperOrigin-RevId: 269350796

src/main/java/com/google/devtools/build/lib/skyframe/PackageLookupFunction.java

@@ -113,10 +113,12 @@ public SkyValue compute(SkyKey skyKey, Environment env)
    */
   public static String explainNoBuildFileValue(PackageIdentifier packageKey, Environment env)
       throws InterruptedException {
+    String educationalMessage = "Add a BUILD file to a directory to mark it as a package.";
     if (packageKey.getRepository().isMain()) {
       PathPackageLocator pkgLocator = PrecomputedValue.PATH_PACKAGE_LOCATOR.get(env);
       StringBuilder message = new StringBuilder();
-      message.append("BUILD file not found in any of the following directories.");
+      message.append("BUILD file not found in any of the following directories. ");
+      message.append(educationalMessage);
       for (Root root : pkgLocator.getPathEntries()) {
         message
             .append("\n - ")
@@ -127,7 +129,9 @@ public static String explainNoBuildFileValue(PackageIdentifier packageKey, Envir
       return "BUILD file not found in directory '"
           + packageKey.getPackageFragment()
           + "' of external repository "
-          + packageKey.getRepository();
+          + packageKey.getRepository()
+          + ". "
+          + educationalMessage;
     }
   }
 

src/test/java/com/google/devtools/build/lib/analysis/AnalysisFailureReportingTest.java

@@ -92,7 +92,8 @@ public void testMissingDependency() throws Exception {
         .containsExactly(
             new LoadingFailedCause(
                 causeLabel,
-                "no such package 'bar': BUILD file not found in any of the following directories.\n"
+                "no such package 'bar': BUILD file not found in any of the following "
+                    + "directories. Add a BUILD file to a directory to mark it as a package.\n"
                     + " - /workspace/bar"));
   }