as default seperator

This makes it obvious where packages are cached and when the cache is
being reused.

Messages are of the form:

===> Fetching recon ({pkg,<<"recon">>,<<"2.2.0">>})
===> Downloaded package, caching as
$HOME/.cache/rebar3/hex/default/packages/recon-2.2.0.tar

and

===> Fetching recon ({pkg,<<"recon">>,<<"2.2.0">>})
===> Version cached at
$HOME/.cache/rebar3/hex/default/packages/recon-2.2.0.tar is up
to date, reusing it

prints requested paths to stdout based on current profile

This patch forces write_file_if_contents_differ/2 to use raw bytes as
when writing files. This fixes an issue where UTF-8 characters are
processed and written back as ISO-8859-1 into the file, which makes it
impossible to use UTF-8 characters in .app.src files.