allow soylink as URI scheme.

- also fix issue in which an empty
  scheme was a valid URI.
- and rename http_link_pattern to uri_link_pattern
  Since the pattern does actually match more than just http-scheme
  URIs it is renamed to reflect the content more precisely.
add function to convert soyfile:// to file://

For autochdir compatibility it will first try to resolve the path
relative to the current namespace; if this fails it will default
back to a path relative to the wikis root directory.

Also add a helper function to determine full namespace path.
add actual call with soyfile utilization.