Support package environment markers

Environment markers can be added at the end of a package requirement
line following a semicolon. For example:

Django==1.5.4; python_version < '3'

Teach the parser to recognize this part of the requirements grammar and
use it to allow duplicate package names that have different
requirements. For example, this is now valid:

Django==1.5.4; python_version < '3'
Django==1.11.18; python_version >= '3'

Also, adjust the "unsorted" rule to allow duplicate package names but
enforce the sort order of their version strings.