Simplify flow type distribution

[TrySound]

In this diff I replaced flow-copy-source with much more simple and less
verbose way to distribute flow types.

I suggest to reuse flow types which already exist in src folder by
reexporting them.

This also requires to publish src.

[FezVrasta]

What's wrong with the current approach?

[TrySound]

It requires unnecessary tool and produces a lot of unnecessary files, which can be reused instead of duplication.

[FezVrasta]

Yes but with the current method we avoid to ship tests and other unused stuff to npm. Also, it's quite common to ship the .flow.js files near the .js files, your approach is way less common.

[TrySound]

Also forgot to mention. Currently flow are placed in es folder where they are absolutely useless. Developers usually import from react-popper not react-popper/lib/es.

[FezVrasta]

If you import from react-popper and your bundler supports ES modules it should read them from lib/es, isn't that the case?

[TrySound]

@FezVrasta .js.flow files are not for bundler. They are consumed by flow. Flow don't care about module field.

[FezVrasta]

Ok so we can simply change flow-copy-source to copy to cjs instead of es, but I'd keep it.

[TrySound]

My way to distribute flow types is like bundling, only one file is placed in dist. Also I'm gonna to propose cjs and esm bundles instead of bunch of babeled files.

[FezVrasta]

I prefer the current approach, really.

About bundles, I don't think there's need to generate them, anyway nobody is going to be able to use them in the browser directly.

[TrySound]

Take a look it's quite common.
https://github.com/renatorib/react-powerplug/blob/master/package.json#L15
https://github.com/souporserious/react-measure/blob/rewrite/package.json#L13
https://github.com/atlassian/react-beautiful-dnd/blob/master/package.json#L37

UMD bundle can be used directly.

[TrySound]

The problem that currently there are two ways to consume the library. With single entry point (import { Popper } from 'react-popper') and with path imports (import Popper from 'react-popper/lib/es/Popper'). Bundling reduces this choice and keep users code consistent.

Also bundling is a way to optimize user bundler work.

[FezVrasta]

We can't provide an UMD bundle because some of our dependencies don't ship UMD bundles.

[TrySound]

UMD usually means bundled dependencies. Only react and react-dom shouldn't bundled in this project.

[FezVrasta]

In any case, that would be an additional dist target, the ones we have should stay as they are.

[FezVrasta]

I fixed the flow-copy-source command in #154.

If you'd like to add an UMD build please open a new PR, thanks!