Add ability to get `input` inside Artifact::fileTags

Hi!

Sometimes, I need to specify Module properties inside Groups. And sometimes, file tags of Artifacts inside Module may depend on this properties.

Example:

^{Product {
        Group {
                files: ["some_file1"]}

                ^{fileTags: ["module.input"]}           

                ^{someModule.outputTag: "some_tag1"}

^}

        ^{Group {}

                ^{files: ["some_file2"]}

            ^{fileTags: ["module.input"]}

            ^{someModule.outputTag: "some_tag2"}

         ^}

        ^{someModule.possibleTags: ["some_tag1", "some_tag2"]
}}

^{// someModule
Module {
    property string outputTag: "default_tag"
    property stringList possibleTags: [outputTag] // Use this property if you specify tags inside a `Group`}

    ^{Rule {
        inputs: ["rtLocalizer.input"]}

        ^{outputFileTags: product.someModule.possibleTags}

        ^{outputArtifacts: {
            return {}

                ^{filePath: input.completeBaseName,}

                ^{fileTags: [input.someModule.outputTag]}

            ^{};
        }
    }
}}

As you can see, I have to use `possibleTags` property, to explicitly specify possible output tags.

But if qbs enable to use input inside `fileTags`, I can simplify the code:

^{Product {}

        ^{Group {
                files: ["some_file1"]}

                ^{fileTags: ["module.input"]}           

                ^{someModule.outputTag: "some_tag1"         }}

        ^{Group {}

                ^{files: ["some_file2"]}

            ^{fileTags: ["module.input"]}

            ^{someModule.outputTag: "some_tag2"}

         ^}

^}

^{// someModule
Module {}

    ^{property string outputTag: "default_tag"}

    ^{Rule {
        inputs: ["rtLocalizer.input"]}

        ^{Artifact {}

            ^{filePath: input.completeBaseName,}

            ^{fileTags: [input.someModule.outputTag]}

        ^}

    ^}
}