Use erl_prim_loader:read_file_info instead of filelib:is_file to make
sure paths inside the escript archive are handled properly.

When rebar ct executes with its default common test directory of "test",
it will generate a hardcoded "logs" directory in every application with
a test directory present, causing an overlap with eunit's test framework
so even test directories with only eunit tests will be processed by ct.

reltool is missing an app resource file before R14A.
Therefore, restore the old check and add TODO.

If running e.g. rebar generate using an OTP development build,
rebar will complain that reltool has the version "", which is
less than the required "0.5.2". This is because rebar_reltool
simply checks the path returned by code:which(reltool), which
doesn't yield version information if used in a development build.

This patch substitutes a more robust method (load reltool and
fetch the info from application:loaded_applications()).

As it happens, this will not be enough to make things work,
but now Reltool will explain that it cannot generate a spec
from a system that is not installed, giving a better hint
as to what needs to be done.