QmlJS: Use canonical paths for matching of import directories

We rely on string comparison for detection of QML import paths. Therefore
make sure that all paths are canonical.

Change-Id: I416bc31915644a888c416d726049668b0e71f29a
Task-number: QTCREATORBUG-12902
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>

src/libs/qmljs/qmljsdocument.cpp

@@ -534,6 +534,7 @@ void Snapshot::insert(const Document::Ptr &document, bool allowInvalid)
 
 void Snapshot::insertLibraryInfo(const QString &path, const LibraryInfo &info)
 {
+    QTC_CHECK(!path.isEmpty());
     QTC_CHECK(info.fingerprint() == info.calculateFingerprint());
     _libraries.insert(QDir::cleanPath(path), info);
     if (!info.wasFound()) return;