Checks a tree of .dxns schemas (Dextrin.Schema.Tree): every file
under SCHEMAS -- files or directories, searched recursively -- read,
every entry named from its path (DXNS.md §6) and compiled in the order
its references need. Every problem is printed as file:line: message
and the task exits non-zero, so CI can run it:
$ mix dextrin.check schemas/
$ mix dextrin.check schemas/ --extern google.protobuf
A name that resolves to nothing in the tree is a problem, unless its
namespace is given with --extern (repeatable): names another tree or
a provider supplies.