Handle multiple requirements for one package

Sometimes a developer needs to install dependencies for several
packages, and each of them has its own requirements.txt file.

Often, the same package is referenced in several files, possibly with
different version requirements.

pip should not print "Double requirement given". Instead, it should
aggregate requirements (e.g., prettytable>=0.6 and prettytable<0.8
turns into prettytable>=0.6,<0.8).