modelbuilder: a directory named `src` triggers more heuristics

modelbuilder: a directory named `src` triggers more heuristics

A MGroup is determined by the fact that a directory is named `src`
Thus, current project that put their module in a src directory can be
correctly identified as MProject

~~~
$ nitls src/nit*.nit examples/mnit_dino/src/*.nit -t
mnit_dino (examples/mnit_dino/src)
|--dino (examples/mnit_dino/src/dino.nit)
|--dino_android (examples/mnit_dino/src/dino_android.nit)
|--dino_linux (examples/mnit_dino/src/dino_linux.nit)
|--fancy_dino (examples/mnit_dino/src/fancy_dino.nit)
|--game_logic (examples/mnit_dino/src/game_logic.nit)
|--graphism (examples/mnit_dino/src/graphism.nit)
`--splash (examples/mnit_dino/src/splash.nit)
nit (src)
|--nit (src/nit.nit)
|--nit_version (src/nit_version.nit)
|--nitdbg_client (src/nitdbg_client.nit)
|--nitdbg_server (src/nitdbg_server.nit)
|--nitdbg_websocket_server (src/nitdbg_websocket_server.nit)
|--nitdoc (src/nitdoc.nit)
|--nitg (src/nitg.nit)
|--nitlight (src/nitlight.nit)
|--nitls (src/nitls.nit)
|--nitmetrics (src/nitmetrics.nit)
|--nitunit (src/nitunit.nit)
`--nitx (src/nitx.nit)
~~~

Signed-off-by: Jean Privat <jean@pryen.org>