Convert --tla/ext-*-file to imports
Convert --tla/ext-*-file to imports

Instead of reading the files directly for --tla/ext-*-file, just add a
code snippet that does an import or importstr.

This makes relative imports work when the code-file path is not in the
current directory. It also makes the import lazy so if the arg/var is
not referenced, the file does not have to exist.

Fixes: google/jsonnet#452

sbarzowski/go-jsonnet,google/go-jsonnet,sbarzowski/go-jsonnet,sbarzowski/go-jsonnet,google/go-jsonnet,sbarzowski/go-jsonnet,google/go-jsonnet,sbarzowski/go-jsonnet,google/go-jsonnet,google/go-jsonnet