This would cause crashes on linux and force people to have a src_dirs
config that is strictly matching what is on the file system rather than
acting as a specification of those that are valid.

To compare, if lib_dirs worked the same, then any repo that did not both
have apps/ and lib/ would crash, as the spec mentions both options as
valid.

- this is done by adding a HiPE dependency
- also in this: adding a test for directories, and fixing some indent

The previous patch at #7c959cc fixed the usage of duplicate values
for directories through relative paths, but mistakenly went overboard
and dropped the `./` path, which is still fairly common. Similarly for
`../".

The code is modified to special-case such values and keep the code
working.

When fetching src_dir values, some relative paths can be inserted. When
deduplicating the paths on the fetch, this fact means that logically
duplicate (but literally different) directories can be returned at once.

By normalizing the names, duplication bugs can be resolved.

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