Use relative symlinks for self links and secondary "packages" dirs.

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index b2dc9b4d3b0366e237c735de9baaaa95242bb6a7..ab65950b0cac7758eaa44bfafeecf48e8538216c 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -232,7 +232,7 @@ class Entrypoint {
       if (entryExists(linkPath)) return;
       ensureDir(packagesDir);
       return createPackageSymlink(root.name, root.dir, linkPath,
-          isSelfLink: true);
+          isSelfLink: true, relative: true);
     });
   }
 
@@ -296,7 +296,7 @@ class Entrypoint {
     return defer(() {
       var symlink = path.join(dir, 'packages');
       if (entryExists(symlink)) return;
-      return createSymlink(packagesDir, symlink);
+      return createSymlink(packagesDir, symlink, relative: true);
     });
   }
 }