Preserve order of requirements

[jwg4]

The reason for this test is the following. If foo_requirement depends only on test_requirement, and both are explicitly installed from files after being mentioned from the same requirements.txt, then pip will not look for test_requirement when installing foo_requirement - it will be clever enough to realize that it's just been installed. But terrarium will complain that it can't find the dependency.

Note that this behavior of pip depends on the two packages appearing in the right order in the requirements.txt file. Doing things this way might be a misuse of undocumented pip behavior, I have no idea. In any case, it is something which pip users might depend on.

[winhamwr]

Thanks so much for the PR with the failing test case!

[jwg4]

I have made a small fix to the test, added a comment, and also made a change which sems to solve this issue. Not sorting the list of requirements in the file means that they are installed in the same order that pip would install them.

[jwg4]

I have checked with the pip project and they say that one should not depend on the order in which the files are installed. pypa/pip#3480 (comment)

Therefore I will try and set up my requirements.txt so that we are not depending on this behavior, in which case they should also work reasonably with terrarium.

However, I believe that this PR is still valid, including the test case. Pip tries to resolve dependencies if these are given in the metadata of a package, and will install in an order so that these are taken into account. Therefore, one should not rearrange the packages from the order chosen by pip.

[winhamwr]

I'm +1 on merging in this PR. We should update the README to make it clear that the order of your requirements affects the hash, though.