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.