Changes to how hex or packages may work in the future will necessarily
bring changes to the format of lock files.

This commit adds an optional framing for future lock files of the form:

    {Version, LockList}.
    <Whatever consultable attributes>

This format is supported such as the LockList is the current lockfile
contents, and will never have more information than it currently does.

Attributes can be whatever and are currently undefined.

Rebar copies will be able to:
- Keep using the core locklist (which avoids breaking the last year or
  so of community libraries using rebar3)
- Warn when it runs an outdated copy in comparison to the lock file
- Automatically rewrite lock files in the format it supports
- Augment or parse files in a version-specific manner.

This changes the usage interface slightly, but is backwards *and*
forwards compatible.