PBS::Setup::Script extends PBS::Setup, gains env and root_dir methods

Signed-off-by: Pedro Melo <melo@simplicidade.org>
When setting up our local::lib-based deps, update PATH and PERL5LIB:

This propagates our deps to any process that is exec'ed from our code, so
that those scripts also have access to our deps.

Signed-off-by: Pedro Melo <melo@simplicidade.org>
Update env() on ::Setup to support "auto" environment, the new default:

If no PROJECT_ENV or :env is defined, we now try to figure out the
PROJECT_ENV to set using this rules:

* if <project_root>/.git exists, set env as "devel";
* if <project_root>/.deployed_env exists, read it and use the first line
  as our env.

If none match, the process dies.

Signed-off-by: Pedro Melo <melo@simplicidade.org>