Make target:filerule able to match package rules

[SirLynix]

When trying to add files with a specific package rule within a package rule:

rule("foo")
    on_config(function (target)
        target:add("files", "bar", { rule = "@package/baz" })
    end)

the rule wasn't found by target:filerule(file), causing an unknown source file error.

This can be easily fixed by using the target:rule(file) method, I think we could replace the whole line with it but I put it as a fallback at the end to not break anything.