Bug fix: Follow only dependencies of current platform

[suutari-ai]

Commit 4900c7c introduced a feature which makes generate_hashes to
generate the hashes of all the available wheel packages for a
distribution. Revert that feature, because it has two problems:

1. Prequ is meant to compile the dependencies for just a single
   platform, the current one. Hashes for any other platform shouldn't
   be in the generated txt file.

2. It broke the PackageFinder so that the dependencies aren't resolved
   for the current platform, but for a magical "can install anything"
   platform and ended up generating incorrect dependency tree. See the
   example below.

Here's an example that demonstrates why dependencies should be followed
for the current platform rather than accepting any wheel package:

Suppose you have "cryptography" in your source requirements and you're
compiling the requirements for Python 2.7. The newest version of
cryptography is currently 2.0.3 which has wheels for Python 2 and 3.

The Python 3 wheel of cryptography==2.0.3 requires asn1crypto>=0.21.0,
cffi>=1.7, idna>=2.1 and six>=1.4.1, but the Python 2 wheel requires
asn1crypto>=0.21.0, cffi>=1.7, enum34, idna>=2.1, ipaddress and
six>=1.4.1. If the Python 3 wheel is used to resolve the dependencies,
then enum34 or ipaddress will not be added to the dependency tree, since
they are not its dependencies. Therefore it's essential to use the
Python 2 wheel when resolving dependencies for Python 2 platform,
otherwise the enum34 and ipaddress will not be pinned in the generated
txt file as they should.

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   80.24%   80.39%   +0.15%     
==========================================
  Files          26       26              
  Lines        1721     1714       -7     
  Branches      347      347              
==========================================
- Hits         1381     1378       -3     
+ Misses        269      267       -2     
+ Partials       71       69       -2

Impacted Files	Coverage Δ
prequ/repositories/pypi.py	90.19% <ø> (+3.04%)	⬆️

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