avoid repeated loading and unloading of modules

like `src_dirs`, `extra_src_dirs` are directories to be copied to
the `_build` dir and compiled. unlike `src_dirs` they are not added
to the .app specification

- The rebar package index files have been moved off the default path and
  will require a new `rebar3 update`
- Caching of downloaded packages automatically takes place in a path
  relative to the CDN used
- The cache path is not shared with hex as we now write and modify data
  in there arbitrarily
- Basic tests plus the working set for more of them is included

rather than profile root

fixes #415

to prevent any data from being overwritten

fixes #395

* fix shell commands relying on non windows shell commands
* fix shell commands using wrong quotes
* implement native wc -l

Adds a `--config` flag to the shell command to specify a config file.
If the flag is not specified, attempts to read the sys_config
defined for relx.

- Solve "Uncaught error" failure in case of syntax error in app.src file.
- Print helpful information on the location of the syntax error.

Changelog since alpha-4:

- rebar3 now bootstraps itself
- app compile fixes on app info contents and config inheritance
- dropping ErlyDTL and going to Mustache for smaller executable sizes
  and faster load time on small CPUs
- dropping `erlydtl compile` command, ErlyDTL task moved to a plugin
- default dependencies are always fetched in the right place and don't
  need to be copied across profile runs
- escriptize fixes
- `unlock` command added
- resolving/avoiding clashes when a rebar3 dep and an application
  dependency conflict together
- profile deduplication (`rebar3 as test,test,prod,test eunit` returns
  the profile/directories for `prod+test`)
- `upgrade` command bugfixes to recognize promotion of transient deps to
  top-level deps
- support template when rebar3 is run as an embedded library
- some optimizations
- parse transform support is better in erl_opts
- fixes to package handling
- fixes to `rebar3 deps`

Whenever files were being locked *and* provided in rebar.config
duplicates could be printed. This is taken care of.

Also whenever atom packages were being used, they would cause the merge
step to fail. The new format does proper name-based deduplication of
resources, giving priority to locks to work.

ModificatioN/needs-update support is still in.

This may also act as a fix for #341, if automation isn't required too
much.

They can be culled and reused in one sort pass.