ProjectExplorer: Implement (Docker)Device::exists(filePath)

While the original plan was to use more specific functions like
'isReadableFile' to cut down the number of accesses, it is
unrealistic to re-write all occurrences in reasonable time.
So make the basic building blocks accessible, too.

Change-Id: I2586ebd19e1e7ae2c884fd2a4180869421293e3a
Reviewed-by: David Schulz <david.schulz@qt.io>

src/libs/utils/fileutils.cpp

@@ -832,7 +832,10 @@ void FilePath::setHost(const QString &host)
 /// FilePath exists.
 bool FilePath::exists() const
 {
-    QTC_ASSERT(!needsDevice(), return false);
+    if (needsDevice()) {
+        QTC_ASSERT(s_deviceHooks.exists, return false);
+        return s_deviceHooks.exists(*this);
+    }
     return !isEmpty() && QFileInfo::exists(m_data);
 }