Make the execution root match the runfiles tree structure for external repositories

One interesting side effect of how this is implemented is that for external
repositories, bin/ and genfiles/ are combined.  External repo output is under
bazel-out/local-fastbuild/repo_name for each repo.

Fixes #1262.

RELNOTES[INC]: Previously, an external repository would be symlinked into the
execution root at execroot/local_repo/external/remote_repo. This changes it to
be at execroot/remote_repo. This may break genrules/Skylark actions that
hardcode execution root paths. If this causes breakages for you, ensure that
genrules are using $(location :target) to access files and Skylark rules are
using http://bazel.io/docs/skylark/lib/File.html's path, dirname, etc.
functions.

--
MOS_MIGRATED_REVID=125095799

1 files changed, 1 insertions, 1 deletions

diff --git a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java
index c96a65e50b..caab4823a1 100644
--- a/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleContextTest.java
@@ -343,7 +343,7 @@ public class SkylarkRuleContextTest extends SkylarkTestCase {
     reporter.removeHandler(failFastHandler);
     getConfiguredTarget("@r//:cclib");
     assertContainsEvent(
-        "/external/r/BUILD:2:10: Label '@r//:sub/my_sub_lib.h' crosses boundary of "
+        "/r/BUILD:2:10: Label '@r//:sub/my_sub_lib.h' crosses boundary of "
             + "subpackage '@r//sub' (perhaps you meant to put the colon here: "
             + "'@r//sub:my_sub_lib.h'?)");
   }