Includes improvments and function documentation for all modules (in
alphabetical order) up to rebar_core, and may have included more in
other modules as I saw fit to dig and understand more of the internals.

Specifically, when fetching an application where the expected hash is
unknown, the hash is validated from the hex index; when the index is
available, the hash is fetched fine and later inserted in the lock file.

However, if the index is not available, the call would simply crash.
This patch fixes thing so that instead, the index is refreshed before
giving up and failing.

The trim_all option used in binary:split/3 is not supported in 17.x.
This patch makes an equivalent operation by eliminating empty split
fragments. From the docs:

    trim
    Removes trailing empty parts of the result (as does trim in
    re:split/3.

    trim_all
    Removes all empty parts of the result.

The new expression is therefore equivalent to the old one, but with the
added benefit of compatibility.

Fixes #1275

This adds support for their format both on the first read and when
looking for modifications to dependencies.

- the internal representation for package locks moves from `{Name, {pkg,
  PkgName, Vsn}, Lvl}` to `{Name, {pkg, PkgName, Vsn, Hash}, Lvl}`
- the internal representation for packages moves from `{pkg, PkgName,
  Vsn}` to `{pkg, PkgName, Vsn, Hash}`
- the hash can be `undefined`, meaning no check will be done
- no checking is done yet.

Instead fetching and resolving src deps (which could depend on pkg deps)
and then pkg deps this patch combines the two into a single set of
iterations by level. The only difference between src and pkg deps in this
new install_deps is how their deps list is found -- from the config or
lock file for src deps and from the neighbors of the vertex for pkg.

Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands

Remove experimental flag from:
* qc compile_only=true
* eunit compile_only=true
* src/foo.app.src.script
* eunit tests=

This allows an <app-name>.app.src.script to be defined and evaluated
when <app-name>.app.src or <app-name>.app are loaded. This allows the
user to add project specific manipulations to app metadata.

This patch adds support for customising the way in which rebar generates
version numbers for app.src files using the `{vsn,Spec}` approach.
Whilst the existing `{vsn,ScmName::atom()}` syntax will continue to
work, users can also pass `{vsn,{cmd,Cmd::string()}}` in which
case the provided *command* will be used. For example:

```erlang
{application, doodah,
 [
  {vsn, {cmd, "git rev-parse --short HEAD"}}]}.
```