Apply `exclude_files` to `files` absolute paths

[danielfinke]

When calling erlfmt with absolute paths for the files to format, the excludes don't apply, even if the resolved paths of the excludes would match the files to format.

Resolve the paths of the excludes based on the current working directory and add them to the excludes.

This issue was observed when using the VS Code Erlang Formatter extension. The extension invokes rebar3 fmt with the full path of the file to format.