attr.output cannot create directories

I would like to have a named output directory from a rule.

`attr.output` can create output attributes, but the outputs must always be files; they cannot be directories. That seems an artificial/unnecessary limitation.

I would like to support directories. Perhaps this could be an argument to `attr.output`. An argument `type` could be `"file"`, `"directory"`, or `"symlink"`.

The issue identifies the attr.output API but names no implementation files or tests. Start by locating attr.output and its existing output-type handling, then determine how directory outputs should behave and verify support for file, directory, and symlink types.

Written by the indexing model from the issue text.