Erlang.mk projects either have a .app.src, or just the Makefile
with an optional .app file for compatibility.

If we have a .app and a .app.src we delete the .app.

If we have a Rebar project with a .app file, rename it to
.app.src and continue with autopatch.

If we have a non-Rebar project, leave the .app file, and then...

For all cases, delete the ebin/ folder after autopatch completes.

This will make sure we don't look twice.

Also make a single call to find the common file formats
rather than many calls.

The initial documentation is a bit rough. It's getting late and
I'd like to have this in sooner rather than later. :-)

The previous approach spawned a shell for every single source files. As
we disable parallel make jobs, this is very time consuming.

The new approach uses an intermediate timestamp file in $(ERLANG_MK_TMP)
to record the last modification of any of $(MAKEFILE_LIST) and touch
all source files in one command. Then, the .app file depends on this
timestamp file.

We test the existence of this timestamp file: if if doesn't exist, don't
touch source files, they will be built anyway.

$(PROJECT).d now depends directly on $(MAKEFILE_LIST); before, this
dependency was indirect through $(ERL_FILES). Also, once $(ERL_FILES)
were touched, we do the same with $(PROJECT).d because there is no need
to regen it because of this.

Error has been detected during the nightly packages build.

After a lot of soul searching and experiments, I am opting
to using curl for fetching files. It works with very little
manipulation on all supported systems. Some might need it
installed, but most come with it.

While wget was working, it had TLS validation deactivated
because some users didn't have CA certificates on their
system or wget was not configured properly. This does not
seem to be a problem with curl as far as I tested, in
particular on OSX.

The fallback to use Erlang was also insecure, and probably
not the best idea. Let's keep things simple. Curl is required.