Allow links to external routes to gather metadata in LinkGenerator::generate

Uploading a patch that makes the proposed change. Since in the LinkGeneratorTest we mock the assembler, we need to change it to return a GeneratedUrl instance, since external URLs are now tested with toString(TRUE) rather than toString(FALSE).

We can see this is an API stable change because the actual output of the link itself is not changed (assuming no other tests break and need fixing).

This seems to make sense, to me. The code had an explicit way to exit early if the link was external by directly creating a generated link. Now external links get generated the same as any others.

I wouldn't expect any tests to be broken as this is very internal.

I don't remember why exactly we had this special case, Not sure if there's a performance aspect to it.

That said, this barely seem relevant to the meta issue because it is about Link objects, which are not used in StreamWrappers, only the underlying Url objects. I suppose it does become an issue if you use FileUrlGenerator::generateUrl() and then pass that Url to a Link object, but there are not that many cases in core that actually do this. It's the last piece in the puzzle.

One more thing to consider here. We might want to deprecate getExternalUrl() only in D11 for D12 as we deprecate the separate interface. Modules that will want to remain compatible with D10 and D11 will need to keep the method anyway and that's going to be confusing otherwise.

Correct me if I'm wrong but this seems like a change that could use a CR?