This will simplify parsing and make it possible to have a uniform lexer for the language. Previously unquoted package names were allowed to include '-'s, which aren't valid identifiers.
In the future, we'll distinguish local paths from packages in the package-manager by looking for a ".roc" suffix, which should only be present in local paths.
Also, refactor out a utility method to format a Collection. This method can currently
replace some of the formatting done in module headers - but the goal is eventually
to be able to replace the code in fmt_list as well, such that there is 'one true way'
to format collections.