Add SearchScope class for --find-links and --index-url related options

[cjerdonek]

This is the first of two PR's to add a SearchScope class to encapsulate the --find-links and --index-url-related options.

The current PR adds the class and stores search_scope on PackageFinder instead of find_links and index_urls. The second PR will change PackageFinder.create() to accept a SearchScope object instead of find_links and index_urls.

This change has a number of benefits, including--

1. Reduces the number of arguments to PackageFinder() and PackageFinder.create(),
2. Further simplifies the PackageFinder class and index.py by extracting more code from it, and
3. Provides a nicer encapsulation of the --find-links and --index-url options, which makes things easier to test and will give us more flexibility in handling those options.

[cjerdonek]

@xavfernandez Pinging you on this as it's very similar to the TargetPython PR you recently reviewed. Thanks if you can take a look!

[pradyunsg]

This is pretty cool! :D

[cjerdonek]

Thanks!