PipRequirement: Add support for VCS, file, index

Closes coala/package_manager#38
5 jobs for pip-repo in 10 minutes and 58 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #76709220
check_code

00:02:25

passed #76709218
test:fedora_25

00:10:48

passed #76709219
test:opensuse_leap

00:10:58

passed #76709217
test:ubuntu_rolling

00:10:47

 
  External
failed continuous-integration/appveyor/branch