Make rebar3 templates check for name clashes

[pablocostass]

This commit changes rebar3 to check for name clashes of the
modules created from its templates with existing Erlang modules
to produce a non-blocking warning for each clashing module.

It does so by checking if any of the modules that will be created
is already defined in the path and, to do so, reimplements the function
code:all_available/0 when rebar3 is used with a version of Erlang/OTP
older than 23.0, as said function was added in that release.

Should close #1386.

[pablocostass]

The CI is failing because I messed up with the conditional definition of the all_available/0 function. Furthermore, I just realized that in order to fix it I would probably have to either:

1. Redefine twice the implementation of that function (the first time if OTP_RELEASE is not defined and the second time if the macro ?OTP_RELEASE outputs a version <= 23)
2. Forget about conditional definitions and always use the reimplementation of code:all_available/0, leaving a TODO note of removing that chunk of code in favor of simply calling the builtin function once it's defined in OTP 23.0

[ferd]

Would it be simpler to translate the name to an atom then calling module_info() on that atom within a try...catch ?

[pablocostass]

Yes, it would be! Why didn't I think of doing that? Thanks for the suggestion @ferd, I'll commit the changes later

[ferd]

just one small change to make it safest

[pablocostass]

Got it

[pablocostass]

@ferd Windows CI crashed while installing Erlang. Can you restart the whole thing? :)

[ferd]

yep I'm watching it