Allow passing proper metadata to Distribution.from_location() and
Distribution.from_filename().
Correct the order of arguments for the importer functions.
Note that some functions accept any iterables, not just sized ones.
Correct the type of the VersionConflict classes' context.
The Python source (Objects/exceptions.c) explicitly checks for null
pointers before using the filename and lineno members. Some libraries,
e.g. pkg_resources, set filename and lineno to undefined values if
indeed none are appropriate.
In Python 3, just as in Python 2, the expected exception argument to
assertRaises() and assertRaisesRegex() must be a subtype of
BaseException, not just of Exception.
Closes#2593
