Change execution root for external repositories to be ../repo

Some of the important aspect of this change:

* Remote repos in the execution root are under output_base/execroot/repo_name, so the prefix is ../repo_name (to escape the local workspace name).
* Package roots for external repos were previously "output_base/", they are now output_base/external/repo_name (which means source artifacts always have a relative path from their repository).
* Outputs are under bazel-bin/external/repo_name/ (or similarly under genfiles). Note that this is a bit of a change from how this was implemented in the previous cl.

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.

Roll forward of bdfd58a.

--
MOS_MIGRATED_REVID=133606309

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 00b27d247e..af79883d2a 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
@@ -340,7 +340,7 @@ 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'?)");
   }