Support recursive imports

[indutny]

Instead of bailing out on code like this:

// root.js
require('./lib').a.b

// lib.js
exports.a = require('./a');

// a.js
exports.b = 1;

We should recognize recursive pattern and store uses in a.js.

[indutny]

It is supported as of webpack-common-shake@1.3.0.

[indutny]

Oh, actually the thing that it supported is more limited:

// a
exports.a = require('./b').b;

// b
exports.b = 1;

If exports.a is unused - exports.b will be removed too.

[indutny]

Recognizing recursive pattern as in the issue description is harder, will keep this issue just in case.