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, 8 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java b/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java
index 5ee730e022..2efdc21693 100644
--- a/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java
+++ b/src/main/java/com/google/devtools/build/lib/buildtool/ExecutionTool.java
@@ -508,8 +508,14 @@ public class ExecutionTool {
 
     // Plant the symlink forest.
     try {
-      new SymlinkForest(
-          packageRoots, getExecRoot(), runtime.getProductName(), workspaceName)
+      SymlinkForest.builder()
+          .setLegacyExternalRunfiles(
+              request.getOptions(BuildConfiguration.Options.class).legacyExternalRunfiles)
+          .setPackageRoots(packageRoots)
+          .setWorkspace(getExecRoot())
+          .setProductName(runtime.getProductName())
+          .setWorkspaceName(workspaceName)
+          .build()
           .plantSymlinkForest();
     } catch (IOException e) {
       throw new ExecutorInitException("Source forest creation failed", e);