ui: add the ability to apply `defaultrc` configs from resources We will want the ability to cat out these resources, but the same would apply to templates, so I'm going to wait for the dust to settle on that. Reading the default config directly from the filesystem is still in place for now. Differential Revision: https://phab.mercurial-scm.org/D7776

ui: refactor `readconfig()` into a form that can consume resources The old form can't completely go away, because files outside of packages still need to be read. The name passed in here is a tuple of `package name, resource` as needed by the resource API. I like the idea of stating the config file is embedded in the executable by listing is as `exe!package.resource`. This would be consistent with how `debuginstall` points to the executable for the python executable, lib, and installed modules. While in practice the filesystem path is available from the backing ResourceReader when the resource is opened, it is a relative path on py2 and absolute on py3. Further, while this would show in the `hg config` output for each option if set as such here, it doesn't show in the `reading from...` line when `--debug` is used. The file isn't actually open where that prints, so there's no way I see to get that info there. So I opted for the simple prefix to distinguish resources from files. Differential Revision: https://phab.mercurial-scm.org/D7775