Improve "No project root found" error message, simplify project root check to only look for package.json

[lencioni]

This error message is too vague and can be confusing for folks. We can
make it better by providing some more context.

More info: Galooshi/atom-import-js#13

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Stop checking for node_modules when finding project root

In #395 we discussed the logic
for determining that something is a project root. It seems that there
are situations where you might have a package.json file but no
node_modules directory (e.g. haven't run npm install yet, or there are
no dependencies). I think we can improve this by simplifying the check
to only look for package.json.

[trotzig]

The new message isn't strictly correct, and it makes me wonder if we could simplify to only look for a package.json file? The reason I didn't is because people might have local packages hidden deep in the tree structure, but I honestly don't know how vommon this is.

[lencioni]

What do you mean by "local packages hidden deep in the tree structure"? Can you give me an example of this?

[trotzig]

Something like

project
  - package.json
  - node_modules
    - ...
  - src
    - foo
      - package.json
      - foo-main.js
    - bar

The foo module here, specifying its main file as foo-main.js.

Never seen it in the wild, but it's valid, right?

[lencioni]

Ah I see. A sub-package that has no dependencies but has other package behavior. Yeah I could see that happening in monorepos. This could even happen if you just haven't run npm install in that directory yet. In any case, I think the existence of a package.json indicates the project root.

[trotzig]

Will you make that change? (Check for package.json only).

The error message in this PR is still better than what is there currently, so I'm fine merging.

[lencioni]

@trotzig PR updated