Support early-stage plugins through `$(DEP_EARLY_PLUGINS)`

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`.

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