Update license_policy_check to support multi-file producing targets

[danielmachlab]

When license_policy_check is applied to a target that generates multiple files, the policy check fails. See below for an example error message.

in target attribute of _license_policy_check rule <:label_for_target_policy_check_is_applied_to> must produce a single file.

This seems to be due to allow_single_file = True in the description of the rule's attr target. This change removes the line, allowing a policy check to be applied to targets producing multiple files or a single file.

This change was tested by applying license_policy_check to a java_binary target (producing 2 files) and a java_library target (producing 1 file). Both policy checks ran successfully.

[aiuto]

I think this needs a BUILD file change too.

[aiuto]

This needs a change to the test case.

(18:56:13) ERROR: package contains errors: examples/policy_checker: Traceback (most recent call last):
--
  | File "C:/b/bk-windows-308l/bazel/rules-license/examples/policy_checker/BUILD", line 47, column 21, in <toplevel>
  | license_policy_check(
  | File "C:/b/bk-windows-308l/bazel/rules-license/examples/policy_checker/license_policy_check.bzl", line 74, column 5, in license_policy_check
  | def license_policy_check(name, targets, policy, **kwargs):
  | Error: license_policy_check() missing 1 required positional argument: targets

target => targets in examples/policy_checker/BUILD