toolcontext: add nit_dir service

Use `NIT_DIR` or some heuristic to find the Nit directory.
In next commits, tools will use it.

Signed-off-by: Jean Privat <jean@pryen.org>
src: adapt tools to use `ToolContext::nit_dir` instead of `NIT_DIR`

Signed-off-by: Jean Privat <jean@pryen.org>
nitunit: learn to use `nit_dir` instead of hardcoding the path of nitg

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update sav related to `cannot find module` errors

The lib directory is not duplicated since the single nitdir player

Signed-off-by: Jean Privat <jean@pryen.org>
mkcsrc: fix a regexp that brokes the reboostrap

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: `nit_dir` as a service

ToolContext exposes a `nit_dir` service that tools can use.
It is so good that, now, the `bin` directory can be directly put in the PATH on most system while `NIT_DIR` is unset.

Previous direct users of `NIT_DIR` are transformed to use the new service.
Also, `nitunit` uses the new service to find the compiler instead of a hard-coded path.

Closes #392
Closes #408

Pull-Request: #412
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>