generalize dependency and config file matching to a common utility and add UI for multiple matching detectors and scripts

[DavidWells]

Look for react scripts in dev and regular deps. https://netlify.slack.com/archives/CGW2Y6XPH/p1554664767295400

https://drive.google.com/a/netlify.com/file/d/1HsmHeJN4fCXFPz_0PeIpBkOYRtSAgMC5/view?usp=drivesdk

This will likely effect other detectors depending on how users setup their dependancies

[DavidWells]

We will probably want to do deeper checks like:

1. Does project have dependancy XYZ, devDependancy XYZ, or peerDependancy XYZ
2. Check for common config files of a project to match
3. Does npm start or npm dev, or npm run build contain a reference to dependency. E.g. Does the build command from netlify.toml/npm scripts contain react-scripts (or any shorthand command of the binary)

This will result in more conclusive matches.

Its possible that more than 1 detector will match. In this case, we should prompt the user to show them a prompt to choose from the matches and then set the type of detector in the [dev] block or console a message for them to do so

[swyxio]

peerDeps doesnt make that much sense. they're for plugins. npm/npm#1400

yes the possibility of multiple matches always made me uncomfortable. i like this idea a lot.

[swyxio]

i changed my mind regarding using a specificity score. it feels like a premature optimization for now. so now we just have this UI to help disambiguate

i have chosen not to implement code to persist selections to netlify.toml dev block as i dont have time but i think that is a natural next step to this

[swyxio]

demo video: https://netlify.slack.com/archives/CGW2Y6XPH/p1554686091311600