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

2 files changed, 3 insertions, 3 deletions

diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/CcCommonTest.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/CcCommonTest.java
index 1fab14fa9b..5b3d20ca95 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/cpp/CcCommonTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/CcCommonTest.java
@@ -696,7 +696,7 @@ public class CcCommonTest extends BuildViewTestCase {
     checkError(
         "test",
         "bad_relative_include",
-        "Path references a path above the execution root.",
+        "../.. references a path above the execution root (..).",
         "cc_library(name='bad_relative_include', srcs=[], includes=['../..'])");
   }
 
diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java
index 2099df68a1..e476138dbe 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/CcLibraryConfiguredTargetTest.java
@@ -1121,8 +1121,8 @@ public class CcLibraryConfiguredTargetTest extends BuildViewTestCase {
   @Test
   public void testSystemIncludePathsOutsideExecutionRoot() throws Exception {
     checkError("root", "a",
-        "The include path '../system' references a path outside of the execution root.",
-        "cc_library(name='a', srcs=['a.cc'], copts=['-isystem../system'])");
+        "The include path '../../system' references a path outside of the execution root.",
+        "cc_library(name='a', srcs=['a.cc'], copts=['-isystem../../system'])");
   }
 
   @Test