The option {recursive,boolean()} can now be set pr directory in
'src_dirs' and 'extra_src_dirs', and on top level in the new
'erlc_compiler' option. Example config:

{erlc_compiler,[{recursive,false}]}.
{src_dirs,[{"src",[{recursive,true}]}]}.

This will cause recursive compilation within the "src" directory, but
not in any other directoires.

Instead of reading every time that rebar_dir:global_cache_dir/1 is
called

Allows overwrite the default cache directory using the environment
variable REBAR_CACHE_DIR.

Tests both absolutes and relative paths.

add tests

note that now ALL `src_dirs` across included profiles are
compiled. previously only the last included profile's `src_dirs`
were used