The "data" attribute doesn't allow files

[judah]

The rules currently don't seem to allow files in the "data" attribute.

Example code: 522dc28

An example with cc_binary succeeds:

$  bazel build tests/binary-with-data:bin3
$ find bazel-bin/tests/binary-with-data/bin3.runfiles -name foo
bazel-bin/tests/binary-with-data/bin3.runfiles/io_tweag_rules_haskell/tests/binary-with-data/foo

But with haskell_binary, it fails:

$  bazel build tests/binary-with-data:binary-with-data
ERROR: /Users/judah.jacobson/repos/rules_haskell/tests/binary-with-data/BUILD:20:10: in data attribute of haskell_binary rule //tests/binary-with-data:bin1: file '//tests/binary-with-data:foo' is misplaced here (expected no files)

Might be as simple as adding an allow_files somewhere, though we'll want to add more test coverage to be sure.