Fix Phoenix project detection

[J3RN]

Spiritual successor to #321

From the original:

The `web' directory is removed with phx1.3 and placed under lib/<app-name>_web instead. Also change phoenix.routes -> phx.routes

My changes:

I changed the name of the alchemist-app-name to alchemist-phoenix--app-name to indicate that it's

1. part of the alchemist-phoenix functionality and
2. a "private" function

For the sake of testing (and I was having great trouble with that), I generated a dummy Phoenix project. This project is admittedly larger than the dummy_elixir fixture, and I'm open to alternate approaches to this. Both projects together are still smaller than the images :smile:

[benbot]

This is great. I was just about to open a PR with the same fix.

Great work!

[baron]

would love to see this merged