skip-prerequisite/stratopan locator/refactor useragent

* Added a --skip-prerequisite feature to help work around
modules that have missing prerequisites.

* The Stratopan locator will now be engaged by default.
I wrote a stub implementation for it.

* Role::FileFetcher has been refactored into Role::UserAgent.
In addition to providing the old FileFetcher methods, it
delegates a subset of LWP::UserAgent methods via an internal
(but global) LWP::UserAgent singleton.  So now you can just
compose the Role::UserAgent into your class, rather than having
to pass a UserAgent object everywhere.