Ignore poorly formed importFrom

Per the discussion at https://github.com/hadley/devtools/issues/407, `document` needs to call `load_all`, which needs a valid `NAMESPACE` file. Because it is not possible to ignore invalid `NAMESPACE` files, prohibit invalid `NAMESPACE` files from being generated in the first place. This pull request ignores and warns on any poorly formed uses of `importFrom` so that using `document` does not create invalid `NAMESPACE`.