Tools should not be in the default metadata gathering

[aiuto]

When you depend on a tool it should not be included in the license/metadata gathering. We have the filtering function, but it is insufficient.

Some ideas:

• filter any dependency in the exec configuration. Crude but mostly right.
• metadata about the attribute usage on attributes. Elegant but hard.

[junglegoog]

Because most tools are executables, attributes referencing such tools can be filtered in the gathering aspects via:

if attr_name in dir(ctx.rule.executable):
  // skip attr