Break out query parsing from setting the query in the model...

...so FindInFilesUI doesn't need to touch the model before the search starts (for #6966).

@peterflynn This approach makes it so we don't have to deal with multiple models. There really wasn't any reason for FindInFilesUI to touch the model before actually starting the search, because all it needed was to parse the query and get error info. One the search starts, it's ok to clear the model at that point (and the existing code already does that in `FindInFiles.doSearchInScope()`).
Yup, that would make sense. `doReplace()` is sort of funny because I wanted to just handle the promise directly in various cases...having a naming convention would make this kind of thing clearer.