Templates now no longer use Make variables for substitution
but instead replace strings with their equivalent:

  template_name: Corresponds to n=template_name
  project_name:  Corresponds to $(PROJECT) or in=project_name

This allows defining templates outside of Makefiles. For
example an external plugin could define their templates
in templates/my_template.erl and then have the following
in the included Makefile:

  tpl_my_template = $(file < $(THIS)/templates/my_template.erl)

By default the created file will be in src/template_name.erl.
This can be overriden with the tplp_* variable:

  tplp_my_template = src/model/my_template.erl

Substitution is applied both to the template contents and
to its path.

In addition, attempting to overwrite an existing file when
creating a template will result in failure.

The tests were waiting for the test group to finish before
they could continue with the next test group. Now "core"
and "all" targets directly depend on individual test cases,
allowing parallel Make to get to the next tests quicker and
removing 1/3rd of the total run time.

  make check -j8 -k  5790,16s user 1207,08s system 627% cpu 18:35,49 total
  make check -j8 -k  6250,13s user 1326,77s system 972% cpu 12:59,16 total

We already have relx-start-stop for that.

And use this in the tests to fetch Rebar only once and then
clone from the local repository. This should speed up tests
a little.

And some refactor and additional testing for overriding.

Turns out we can override some variables even if ?= is not used,
so nothing had to be done to support overriding.