Bug: xo ignores package.json config when run from a project subfolder

[teehemkay]

When xo is run from a project subfolder, it ignores the config in package.json.

This is regression introduced between 0.26.1 and 0.27.0 (likely in #416)

How to reproduce (see my SSCCE here):

• setup xo with a config minimally different from the default one (e.g. use spaces instead of tab)
• create a foo/bar.js file that uses spaces instead of tabs
• from the top-level project folder (the one that contains package.json) run:

$ xo foo/bar.js

• observer that there are no error or warning
• now run the following commands:

$ cd foo
$ xo bar.js

• observe the following error message:

bar.js:2:1
  ✖  2:1  Expected indentation of 1 tab but found 2 spaces.  indent