Renaming "gots" didn't go well. I think the current naming is the most concise way to put it. If we rename it to "got", then the loop iterator can't be called "got", and the simple relationship between the iterator and collection names is lost. "Actual" and "actuals" are the other names that occurred to me, but they look so much like "allowed" that the code becomes harder to read.
* Add --require-hashes option. This is handy in deployment scripts to force application authors to hash their requirements. It is also a convenient way to get pip to show computed hashes for a virgin, unhashed requirements file. Eventually, additions to `pip freeze` should fill a superset of this use case.
* In --require-hashes mode, at least one hash is required to match for each requirement.
* Option-based requirements (--sha256=...) turn on --require-hashes mode implicitly.
* Internet-derived URL-based hashes are "necessary but not sufficient": they do not satisfy --require-hashes mode when they match, but they are still used to guard against transmission errors.
* Other URL-based requirements (#md5=...) are treated just like flag-based ones, except they don't turn on --require-hashes.
* Complain informatively, with the most devastating errors first so you don't chase your tail all day only to run up against a brick wall at the end. This also means we don't complain that a hash is missing, only for the user to find, after fixing it, that we have no idea how to even compute a hash for that type of requirement.
* Complain about unpinned requirements when hash-checking mode is on, lest they cause the user surprise later.
* Complain about missing hashes.
* Complain about requirement types we don't know how to hash (like VCS ones and local dirs).
* Have InstallRequirement keep its original Link around (original_link) so we can differentiate between URL hashes from requirements files and ones downloaded from the (untrustworthy) internet.
* Remove test_download_hashes, which is obsolete. Similar coverage is provided in test_utils.TestHashes and the various hash cases in test_req.py.
