Rather than doing a check on each single file in a dep set (which can
grow quite large), do a dynamic rebuild of deps based on whether the DAG
mentions the files have been modified. To avoid reloading the whole DAG
more than once and then having to still map files, the compiler version
is added to rebar_compiler_erl's CritMeta for the DAG.

This comparison is only set in place to impact the
rebar_prv_install_deps call that handles pruncing and culling deps by
the compiler provider. Intra-DAG comparisons still need to be done by
rebar_compiler_erl for things to work, and the hope is that since most
built-in compiler mods end up generating erl files in deps, that will be
transparent enough to work well.

A DAG call to check the status of the dag (vsn, critmeta, existing,
etc.) has been added to do the faster check there and allowing proper
dispatch.

Plugins couldn't be updated because they don't really use the standard
dep-style facilities for things due to an early filtering.