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.