do not output when referencing

[bassjobsen]

should fix #2464

[lukeapage]

Any chance of adding tests?

[lukeapage]

and please can you merge latest so the builds pass?

[bassjobsen]

oh i seems i commented on the wrong issues earlier today, sorry. For this issue (output when referencing) i tried to add a test. After all i focused in the wrong issue i think, cause @import (reference) "*.css" makes no sense in the first place.

Now this PR makes sure that @import "*.css" is not outputted when referenced.

When writing the test a found that only changing the Import.prototype.genCSS in less/tress/import.js caused an newline on the place of the @import in the output. For the latest reason i have add the same kind of logical in to-css-visitor.js too, as follows:

    visitImport: function (importNode, visitArgs) {
        if (importNode.path.currentFileInfo.reference !== undefined && importNode.css) {
            return;
        }
        return importNode;
    },

In fact only the changes in to-css-visitor.js are required, but for me it feels unnaturally do not the same in less/tress/import.js