Utils: Add FilePath::symLinkTarget

And implement it for the docker device.

This replaces the previous unused and not really implemented
FilePath::resolveSymLinkTarget.

Change-Id: I9dcb4f8276dbb88b21959276da0d50135742fba0
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/fileutils.cpp

@@ -391,13 +391,6 @@ FilePath FilePath::resolvePath(const QString &fileName) const
     return FilePath::fromString(QDir::cleanPath(toString() + QLatin1Char('/') + fileName));
 }
 
-FilePath FilePath::resolveSymlinkTarget() const
-{
-    // FIXME: implement
-    QTC_CHECK(false);
-    return *this;
-}
-
 FilePath FilePath::cleanPath() const
 {
     FilePath result = *this;
@@ -1009,6 +1002,19 @@ bool FilePath::needsDevice() const
     return !m_scheme.isEmpty();
 }
 
+/// \returns an empty FilePath if this is not a symbolic linl
+FilePath FilePath::symLinkTarget() const
+{
+    if (needsDevice()) {
+        QTC_ASSERT(s_deviceHooks.symLinkTarget, return {});
+        return s_deviceHooks.symLinkTarget(*this);
+    }
+    const QFileInfo info(m_data);
+    if (!info.isSymLink())
+        return {};
+    return FilePath::fromString(info.symLinkTarget());
+}
+
 
 /// Find the parent directory of a given directory.