templater: replace templatepath() with function that also opens the file For frozen binaries, such as those created by PyOxidizer, I plan to make it so the templatespec can keep an opened file/resource to read from instead of needing a file path. Having `templatepath()` return an opened file should help with that. At this point, it's just a wasteful extra opening of mapfiles that we'll open again later. I'll update the read-side next so it reads from the file-like object without opening the file again. Differential Revision: https://phab.mercurial-scm.org/D8892