--pre flag should affect the finder and not the requirements

Storing the --pre flag on the finder enables easily being able
to have it affect all package discoveries made with that finder.
The previous method of passing it into the InstallRequirement
meant that only top level dependencies were controlled by
--pre
Merge pull request #1040 from pypa/use-pre-for-dependencies

Fixes #1038 --pre flag should affect the finder and not the requirements