An opt-in way to warn on include

[jchavarri]

Using include in melange builds can be problematic, as any definitions in the included modules get duplicated. If this happens inside a submodule, where all these definitions are put into a JS object, it can have an impact on bundle size, as bundlers won't be able to dead-code eliminate any of them.

Example

I wonder if there could be a way to optionally enable some warnings on melc when include is used, explaining this problem in detail.