Relative paths to .packages files were handled incorectly.

This makes paths to .packages absolute before trying to resolve paths
that are relative to it. It depends on:
  https://fuchsia-review.googlesource.com/c/16962/

Change-Id: I2c4682d3f3ebd2e36feffa3c7d4dacf4ec533882
diff --git a/file_loader/file_loader.cc b/file_loader/file_loader.cc
index 0ff3102..8660945 100644
--- a/file_loader/file_loader.cc
+++ b/file_loader/file_loader.cc
@@ -137,8 +137,8 @@
     return std::string();
   if (package_path.find(kFileURLPrefix) == 0u)
     return package_path.substr(kFileURLPrefixLength) + library_path;
-  return files::GetDirectoryName(packages_) + "/" + package_path + "/" +
-         library_path;
+  return files::GetDirectoryName(files::AbsolutePath(packages_)) + "/" +
+         package_path + "/" + library_path;
 }
 
 std::string FileLoader::GetFilePathForFileURL(std::string url) {