Add argument to tag() such that the resulting rule is public

[SpangleLabs]

By default, the tag() function creates a hidden build rule, which cannot be detected with plz query filter and the like.

It would be helpful if there was a boolean option to tag which would allow creating non-hidden rules, as we're otherwise doing a lot of string concatenation of target names manually, which is inconsistent

[sellitforcache]

I would also like this.