hgweb: parse WSGI request into a data structure Currently, our WSGI applications (hgweb_mod and hgwebdir_mod) process the raw WSGI request instance themselves. This means they have to talk in terms of system strings. And they need to know details about what's in the WSGI request. And in the case of hgweb_mod, it is doing some very funky things with URL parsing to impact dispatching. The code is difficult to read and maintain. This commit introduces parsing of the WSGI request into a higher-level and easier-to-reason-about data structure. To prove it works, we hook it up to hgweb_mod and use it for populating the relative URL on the request instance. We hold off on using it in more places because the logic in hgweb_mod is crazy and I don't want to involve those changes with review of the parsing code. The URL construction code has variations that use the HTTP: Host header (the canonical WSGI way of reconstructing the URL) and with the use of SERVER_NAME. We need to differentiate because hgweb is currently using SERVER_NAME for URL construction. Differential Revision: https://phab.mercurial-scm.org/D2734