By default rebar3 displays compiler sources as absolute paths in their
original location, which is under the build dir.

This change introduces an option 'compiler_source_format' to format
sources in two alternative ways:

    relative
    absolute

When either 'relative' or 'absolute' are specified, the file is
resolved to its original location when it is a link. When 'relative'
is specified, the path is displayed relative to the current working
directory. When 'absolute' is specified, the path is absolute.

The default value is 'unchaged' which leaves the compiler source
unchanged.

This is arguably too flexible as I suspect most people would opt for
'relative' all the time - it's the most compact representation of the
file and is sufficient to find the source given cwd. The change
however is meant to introduce the change gradually, preserving
existing behavior and giving users a choice for formats.

In time perhaps the default can be changed to 'relative' - but still
allowing users to revert to the other two options ('absolutel' and
'unchanged') as needed.

- robocopying a directory into another directory recursively expects the
  directory name to be properly mapped onto the destination, otherwise
  all the files are copied into the given path. This patches things so
  a directory-to-directory robocopy works as expected in a linux mindset
  so tests pass
- the test for canonical paths didn't expect a windows environment at
  all; the test (and library) is modified to be consistent in that
  environment: always with a native format and with proper support of
  drive letters.

* modify compiler interface to work on either application objects or
  directories containing source files
* compile all sources in `src_dirs` to the application `ebin` dir and
  all sources in `extra_src_dirs` to a directory mirroring it's
  position in the app's `_build` directory. for example, `apps/foo/more`
  would compile to `_build/default/lib/foo/more`
  for `extra_src_dirs` in the root of a project with multiple
  applications (so orphan directories that don't "belong" to an
  application) compile to `_build/default/extras/more`
* copy directories specified in `extra_src_dirs` into the `_build`
  directory so tools like `ct` and `xref` that expect source to be
  in a particular location still work
* clean compiled artifacts from all `extra_src_dirs`
* alter `eunit`, `ct` and `cover` to work with the new directory
  structure
* billions of new tests

`reduce_path/1` -> `canonical_path/1`
`relative_path/2` -> `path_from_ancestor/2`

add tests

swap `file:list_dir` for `file:list_dir_all` due to nonexistence of latter prior to r16x

(in `rebar_file_utils`)