try to read sys.config when running `rebar shell`

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.

Manage syntax error in app.src files.

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

switch mustache imlementation to https://github.com/soranoba/mustache

fix }}} bug in mustache

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`

Use resource handlers to deal with deps/semver

handle missing package in registry by skipping

Fix dupes in deps provider, support atom pkgs

Kovyl2404 app discover profile duplication

include relx templates in escript

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.

on upgrade of git branch check against remote

upgrade relx dep to latest mustache branch commit

Remove erlydtl and replace with mustache

enable parse transformations in rebar config

Conflicts:
	test/rebar_profiles_SUITE.erl

don't load package registry unless there are pkg deps to solve

include _checkouts in release lib dirs if it exists

read in application details into app_info after fetch

Avoid topsorting deps twice

They can be culled and reused in one sort pass.

support templates even when not run as escript

handle transitive deps being promoted

merge_locks crashed if a config didn't have a deps entry