It's better than extending help:: because this gets printed
at the end instead of before Erlang.mk's own help.

If the application's Makefile specify either:
    DEP_PLUGINS = $(PROJECT)
or e.g.:
    DEP_PLUGINS = $(PROJECT)/mk/dist.mk
then load the plugin from the application instead of a dependency.

This helps when you have an application with common Erlang modules and
Erlang.mk plugins: your common application can load Erlang.mk plugins
exactly like other applications depending on the common application.

Regular plugins (`$(DEP_PLUGINS)`) are loaded near the end of Erlang.mk.
This is fine when you want to modify variables initialized earlier in
Erlang.mk or add new targets and variables.

However, it doesn't allow you to declare more dependencies because they
are loaded too late for that.

This commit introduces a new variable, `$(DEP_EARLY_PLUGINS)`, which can
be used to list plugins meant to be loaded near the beginning of
Erlang.mk. Those allow to append to the list of dependencies.

They work exactly like regular plugins otherwise. The default filename
loaded is `early-plugins.mk`.

This will allow to provide permanent URLs to the documentation.

Plugins can automatically be fetched and included from
dependencies. All that is needed is to add either the
dependency name or the name + path of the plugin to the
DEP_PLUGINS variable.

Useful for allowing tools to easily add support for new
targets, adding templates or for putting the whole build
ecosystem in one common dependency.