Use a relative path for the runfiles tree

Add preconditions to enforce this and remove some now unnecessary code.

A small step towards #1593.

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PiperOrigin-RevId: 150625693
MOS_MIGRATED_REVID=150625693
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RunfilesSupplierImpl.java b/src/main/java/com/google/devtools/build/lib/analysis/RunfilesSupplierImpl.java
index 79ecce1..a863234 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RunfilesSupplierImpl.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RunfilesSupplierImpl.java
@@ -65,6 +65,7 @@
       PathFragment runfilesDir,
       Runfiles runfiles,
       @Nullable Artifact manifest) {
+    Preconditions.checkArgument(!runfilesDir.isAbsolute());
     this.runfilesDir = Preconditions.checkNotNull(runfilesDir);
     this.runfiles = Preconditions.checkNotNull(runfiles);
     this.manifest = manifest;