Linting: Restrict WordPress import path access

[aduth]

Related: #941, #929

This pull request seeks to add a lint rule restricting path access for WordPress dependencies. For example:

// Good:
import { keycodes } from '@wordpress/utils';

// Bad:
import { ENTER } from '@wordpress/utils/keycodes';

Implementation notes:

See also:

• http://eslint.org/docs/developer-guide/selectors
• http://eslint.org/docs/rules/no-restricted-syntax
• http://astexplorer.net/#/gist/d349d521ab8ddeaa092db7374af72e42/1aa94d1abe644d79da1c8ebd08aae08d43141e59

I found the selectors syntax to not be particularly happy with escaping slashes (\\/) except when using the unicode escape (\\u002F).

Testing instructions:

Verify there are currently no lint issues:

npm run lint

Introduce a lint issue by finding a WordPress dependency and suffixing a path, then re-run the command above and verify the lint issue is surfaced as an error.

[codecov]

Codecov Report

Merging #2211 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2211      +/-   ##
==========================================
+ Coverage   23.05%   23.14%   +0.08%     
==========================================
  Files         141      142       +1     
  Lines        4372     4481     +109     
  Branches      739      770      +31     
==========================================
+ Hits         1008     1037      +29     
- Misses       2840     2898      +58     
- Partials      524      546      +22

Impacted Files	Coverage Δ
blocks/api/parser.js	96.22% <0%> (-0.92%)	⬇️
editor/index.js	0% <0%> (ø)	⬆️
editor/inserter/index.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/block-list.js	0% <0%> (ø)	⬆️
blocks/library/text/index.js	41.17% <0%> (ø)
components/button/index.js	93.75% <0%> (+2.84%)	⬆️

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[nylen]

What happens when we begin requiring npm packages like @wordpress/hooks? Path imports should be fine there.

Do we want to match our list of Webpack entry points specifically, or keep this restriction for all @wordpress imports for consistency?

[aduth]

The need to traverse into path structure of a dependency is few and far between. I'd be more concerned about all of the places we have this pattern repeated throughout the codebase, particularly if/when we add/remove from the list.