Schema paths with only package name won't load

[StokesMIDE]

In SCHEMA_PATH, package-relative paths containing only the package name don't work; even though idelib.schemata is a valid, importable submodule, "{idelib.schemata}" doesn't work. It will work if followed by a slash ("{idelib.schemata}/"), however. This works, but it would be nice if the package names could be kept 'pure', as opposed to a combination of a module name and a filesystem path.

The simplest fix would be to have core._expandSchemaPath() include a slash if the path ends with a brace.