parse
Parses name - a file name with its suffix already off, or a class's simple name.
Parameters
name
The bare name, V<version>__<description> or R__<description>.
origin
What to call this in a refusal: the path it was found at, or the class's full name. The message has to name something the reader can go and open.
Throws
INVALID_MIGRATION for anything that is not one of those two shapes.