Fix runfiles creation for directory based runfiles.

tools/runfiles/runfiles.rs

@@ -141,6 +141,10 @@ impl Runfiles {
  };
 
  let repo_mapping = raw_rlocation(&mode, "_repo_mapping")
+ // This is the only place directory based runfiles might do file IO for a runfile. In the
+ // event that a `_repo_mapping` file does not exist, a default map should be created. Otherwise
+ // if the file is known to exist, parse it and raise errors for users should parsing fail.
+ .filter(|f| f.exists())
  .map(parse_repo_mapping)
  .transpose()?
  .unwrap_or_default();