The new URL built-in that will reside in URL.{h, cpp} will have an
URLSearchParams member, which means it has to include its header, and
as such URLSearchParams.h can't include URL's header, which it needs as
it uses the url_{encode, decode} functions.
This namespace will be used for all interfaces defined in the URL
specification, like URL and URLSearchParams.
This has the unfortunate side-effect of requiring us to use the fully
qualified AK::URL name whenever we want to refer to the AK class, so
this commit also fixes all such references.
