Remove workspaces

[ericclemmons]

Before #28 or #21, we can fix #14 by:

1. Removing yarn workspaces.
2. Keep yarn link for polydev.
3. Move each package into examples with a package.json.
4. Move packages/polydev to src.

Running an example should require cd into that directory, yarn and polydev.

By doing this, we can ensure that hoisted modules don't affect development/examples.

Also, np will behave as expected.

• Use execa, because shelljs doesn't support colors.
• delete require.cache[handlerPath] still needs to be fixed: when HMR fails, we need to delete the cached version. The problem with clearing it every single time is that the hits++ example breaks when we clear the cache. Once an error occurs, file can't be updated sidorares/hot-module-replacement#15