Investigate reimplement using source plugin

[guibou]

Source plugin may solve a few issues at the same time:

• No more template haskell (I don't consider it a problem, but people sees it as a deffect)
• No need to haskell-src-exts and haskell-src-meta anymore. This will dramatically reduce the dependency footprint. Apparently that's a reason of the lack of adoption of PyF.

[guibou]

On the other hand, people will need to setup source plugin for their build. I need to investigate how this is a problem when using something different that cabal or stack.

[carlosdagos]

:+1: if this could remove the hakell-src-exts dependency.

Thanks!

[guibou]

Now that haskell-src-exts is not a dependency anymore, the source plugin implementation may just be an option in order to get ride of template haskell