Move HTTPRequest.url to the main library

[guoye-zhang]

Resolves #76

[guoye-zhang]

Are there ways to re-export these symbols so we don't need to introduce API breakages?

2 breaking changes detected in HTTPTypesFoundation:
  💔 API breakage: var HTTPRequest.url has been removed
  💔 API breakage: constructor HTTPRequest.init(method:url:headerFields:) has been removed

Maybe @_exported import or @_disfavoredOverload?

[Lukasa]

@_exported import is totally workable.

[guoye-zhang]

@_exported import doesn't seem to address the API breakage check. Maybe the check itself is too strict?

[Lukasa]

API breakage is usually strict, and I think this is one way in which it is.