Tolerate text from dub before start of json

JSON parsing moved to `fldd--get-dub-package-dirs-json` so that
`fldd--get-dub-package-dirs` doesn't have to think about it and the tests
don't have to call `json-read-from-string`.

`fldd--get-dub-package-dirs-json` strips any text prior to the first
opening brace character so that dub's prattle about installing modules
is tolerated.