Commit 029cbcc7 changed the return value of `rebar_git_resource:lock_/2`

None of the version checks are considered safe enough by mix
maintainers, and so the agreed upon mechanism is to just set this
environment variable, which will be supported implicitly by rebar3
versions that can handle it.

This lets build artifacts (aside from some priv/ issues with plugins) to
properly be located within their build folder rather than the deps
folder, as the --outdir switch would otherwise have done.

Not all of the changes to the git resource were reverted when
the sparse checkout support was moved to a new resource,
git_subdir_resource. This fixes issues caused by these changes
that were accidentally left.

Since the compiler refactor changed the order of steps and all hooks get
applied at once, knowing which app is being escriptized isn't really
obvious, particularly when many apps individually define their own 'main
app' and they get run at once.

This adds the name of the main app being run when escriptizing it to make
it clear what goes on, especially if there are failures.

This was reported in https://github.com/erlang/rebar3/pull/2211#issuecomment-615937628

Fixes #2273

This patch aims to make rebar3 delete a dependency by first looking for
its name (fixing `del` on hex dependencies) and if that isn't found
remove it as always, keeping its current behaviour with the `del` override.

Starting with OTP-22, Erlang started changing how volume names are handled in
windows: filename:join/1 drops drive letters, filename:split/2 and
file:absname/1 use c:/ as a form instead of c:\\

We adjust the file_utils and their tests accordingly.

Rebar3.14 has new tests that were not windows ready and those are fixed.

Relx has two failing tests that are not covered in this branch.

Tested in Windows 10 with Powershell.

This reverts commit b7cfa757dc.

We run the analysis for an extra_src_dir by making a fake app for it,
but send in the app alone for analysis.

In there, the DAG pruning routine looks at the DAG and files submitted
and goes "this right here is a project with 99% of its apps deleted". It
then tries to prune the whole DAG except for some test files. The code
"works" simply because there's a false-positive check that makes sure
the file is on disk before removing it for the DAG.

This ends up making extra runs where ~80% of the time is spent
double-checking the false positives for file deletions.

This commit fixes this by merging in all extra_src fake apps and making them
run in a single analysis phase, meaning we only pay the cost of the DAG
pruning once for the whole project, making it faster than any sparse
repo.

There's also a small patch needed for the root-level extra src dirs;
turns out that since the context-handling in the `rebar_compiler` uses a
map to store content, running single-pass analysis clobbered entries for
a given app if they had more than one extra_src_dir in there.

I also took the time to clean up the ordering of that file.

Into a standalone module where it can be reused for optimisation work
later on.

Cleans up some annoying broken abstraction.

Also fix erl_first_file artifact tracking, which will avoid seeking to
disk by finding the opts in the DAG

This reverts commit f0c8c202af.

Because the resource doesn't absolutely require using a sparse
checkout, it only matters that rebar3 knows the correct path of the
application, the resource is named git_subdir.

It has to be git specific because the reason this subdir support is
special is the need to keep the code checkout as a usable git clone
to run commands against. This means the resource can't just copy
the content of the subdirectory to the application out directory.

End result should allow to look like this:

    $ head -n 3 rebar3
    #!/usr/bin/env escript
    %% Rebar3 3.14.0-rc1
    %%! +sbtu +A1

This will allow tools like mix to read the top of the file to know the
version being used in the current path (when using features that aren't
forwards-compatible) without having to invoke the tool itself.

The version (full dynamic one) is also added to the local install:

    $ head -n 2 /home/ferd/.cache/rebar3/bin/rebar3
    #!/usr/bin/env sh
    ## Rebar3 3.14.0-rc1+build.4599.ref6b0f90d

The `##` comments are used to keep a fixed width over the other form.

To be backported to 3.13.2 as well.

This commit switches the relx dependency to branch 4.0.0. It is the
first iteration of integration with relx 4.0 which adds many changes
to how relx works and how the two integrate.

rebar_relx module now creates a relx state from the relx configuration
and pass it along with all the OTP applications rebar3 knows about
which may be used in the release. This allows relx to not have
to duplicate the effort of application discovery like before.

Since this is intended to be released only when OTP-23 is released
the testing of OTP-18 has been removed.

- topsort of an app does a full analysis of all apps to come up with
  their app-level DAG
- each extra_src_dir has its own analysis run
- each extra_src_dir run is simulated as being an app
- we therefore end up doing a lot of useless analysis for single app
  runs

This patch notes runs with 1 or 0 apps and skips doing the topsort
analysis on them, which speeds things up further.

Would close erlang#2202. In case of no apps found when calling
`rebar_state:project_apps(State)` rebar3 now throws an error with
a warning.

Closes #2108. Changes the path to be printed to a relative path
to the current directory.

Also removes the `exports_not_used` warning from the default ones,
as Tristan suggested.

Also fixes artifacts left on filesystem, and produces less
filelib:last_modified() checks.

now all sources have at least one edge with 'artifact' label,
let's ignore it when choosing whether to compile in parallel
or sequentially

The propagation was confusing source files and artifacts; the artifact
ordering was flipped, and the tagging non-mandatory (aside from as
edges), which made things hard to identify when plugins for compilers
are used.

Now it should work in Erlang/OTP versions earlier than 20 and
update the symlink in case of one already existing (so that `last`
always points to the last run).

This patch enables two features:
 * in rebar.config, {diazlyer, [{plt_apps, all_apps}]} makes rebar3 to
   add all project files into PLT for subsequent analysis
 * rebar3 ct --app=myapp allows single application success typing

This allows workflow suitable for large codebases:
 * "rebar3 ct dialyzer --succ-typings=false" to build PLT with all apps
 * "rebar3 ct dialyzer --app=myapp" to perform success typing of myapp

Implements #1734. In case of not being able to create the
symlink rebar3 warns through `?DEBUG`.