Clean up JS .import/.pragma directive scanning

Clean up JS .import/.pragma directive scanning

There's a scanner in QQmlJS::Lexer::scanDirectives that can parse those, so
let's get rid of extra parser that operates on a string. Instead this way we
can do the scanning all in one shot, avoid detaching a copy of the source code
string and (most importantly) bring the parser closer to the copy in Qt
Creator, which uses the directives approach to extract imports and pragma.

Change-Id: Iff6eb8d91a45d8a70f383f953115692be48259de
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>