Route constructor that routes incoming requests if their name has an
expected prefix.

...e. g. (prefixed-request-name-route "auth.log-in" ".") will only accept a
request if its name starts with "auth.logIn" (CamelCase will be hyphenated).
The request will be routed to a target the name of which equals the last
segment of the request's hyphenated name, e. g. the reuest name
"auth.logIn.logIn" will be routed to a target named "log-in".
Use a prefixed route for the "File Upload" service.
Convert the given expected prefix to a string to make it work with
symbols, too.