net/url: support HTTP/1.1 connections
raco pkg: add support for local package-index databases

Adds `pkg/pnr-db', `raco pkg index-copy', and `raco pkg index-show'.

Includes tools to build a database of modules that are supplied by
packages, which will be useful for a tool to recommend package
installs when a module is not found.

Also, document `pkg/lib' and add extra helper functions
for getting package information from a package name resolver.
raco pkg: support platform-specific package dependencies

A platform-specific dependency is useful for triggering
installation of a platform-specific library only on the
platform where its needed.