Fix bad module resolution (#42)

[VeselovAlex]

Replace require.resolve('packagename'), which looks for packages relative to markdownlint-cli root, with require.resolve('packagename', {paths}), where paths are calculated relative to working directory

[VeselovAlex]

Why does is fail on AppVeyor? Looks like it does not copy package from test\custom-rules\relative-to-cwd\node_modules

[DavidAnson]

AppVeyor uses Windows, if that is not what you use, maybe there is a path issue there?

Regarding the _ APIs, aren’t those undocumented and unsupported? That worries me, as does the Node 8 requirement. I would love if we could figure out a way to get the right behavior on Node 6 with public APIs.

[VeselovAlex]

Node 8 is not required. require.resolve.paths and require.resolve(module, paths) are added to public API in Node 8, so we can use them to search packages relative to working directory. For Node 6 we can use Module._resolveFilename(filepath, {paths: paths}) as a fallback.

To replace Module._nodeModulePaths(process.cwd()) we can create custom function to build node resolution path list, but should we? This method is available even in 0.10.25 and was not modified for at least 2 years, so it seems to be stable enough

[DavidAnson]

That’s fair, thank you. I’ll have a look at this soon for a new release. I do want to understand the CI failure, though.

[VeselovAlex]

Tests fixed, seems to an issue with execa on Windows

[DavidAnson]

Yay!!

(Did you mean to leave that console.log in the code?)

[DavidAnson]

I should be able to accept this PR tonight. I’ll also update the markdownlint dependency and investigate the other open issue. Once done, I’ll publish a new version. Thanks for the help!