1.0.0-alpha5

Another month, another alpha. This time with some important improvements:

• #89 fixed some serious performance problems. It's still not awesome, mostly due to Composer making a lot of unnecessary requests (which is an upstream problem; see composer/composer#11704), but this is a big improvement. There was also #87, which reduced unnecessary network traffic.
• #86 removed an old hard-coded assumption that was getting in the way of real-world use.
• #88 and #84 fixed problems that broke integration with packages.drupal.org.
• #90 made testing a tad easier. 😎