Ensure that *only* binder/ subfolder is used if it exists

Right now our buildpack logic can sometimes break. Here’s what’s up:

What we’d like

If a binder/ folder exists in a repo then it will be used in the buildpack process. Currently this is checked by using the binder_path method (link) within each detect call.

Where this goes wrong

If a repository has a binder/ folder with config files in it, but there is a higher-priority config file in the repository root, then that file will take precedence and short-circuit all subsequent config file searching (e.g., having a myproject/binder/requirements.txt file, but also a myproject/Dockerfile).

How we could fix this

Pass a prefix to the “detect” method that’d define a specific folder in which to look for config files. Then just before here we could check whether we need to be searching in a binder/ folder, and if so then pass that to the detect method as a prefix for all subsequent buildpacks.

cc @yuvipanda - if you think that’s a reasonable path forward then I can make a PR

related to #82

[willingc]

It’s cool to leave here. I just want to make sure that a contributor doesn’t try to submit a PR @choldgraf.

[choldgraf]

yep - only reason that I haven’t closed yet is because I think it’ll be closed by #277 in repo2docker…I think that I was overestimating the scope of work that’d need to be done to fix this. I’m happy to close this now if you prefer, instead of waiting for that PR to land