Internal helper wrapping system.file() for files under
inst/templates/. It exists for the error message rather than the lookup:
system.file(mustWork = TRUE) reports "no file found" when the package is
installed and "Can't find package file." under pkgload::load_all(),
neither of which says which file was missing or where it was expected.
During development that is the difference between a five-second fix and a
traceback.