Chore: set package.json's main to dist

[sohkai]

The main field should probably be pointing to the built version, rather than the original src. Without this, some module bundlers (and uglify) will choke.

[daviddias]

@sohkai are you using the latest uglify-es? The issue is that uglify2 didn't support ES6, uglify3 will do it, it is what we use to get that dist version https://github.com/ipfs/aegir/blob/master/package.json#L81

[sohkai]

Even so, I'm of the belief that the main output should always be the transpiled version so that end users don't have to build it themselves.

Getting tree-shaking from import can be done using the pseudostandard module field.

[daviddias]

Closing in favor of tracking this discussion at ipfs/js-ipfs#938 The solution won't require this step. //cc @vmx