Utils: Replace FilePath::onDevice() by new FilePath::withMappedPath()

Basically a.onDevice(b) == b.withNewMappedPath(a), matching the order
of b.withNewPath(a).

Whether the (curretly docker-specific) path mapping is useful /there/, and
whether some of the calls are needed at all is dubious. I added some
FIXME and changed a few cases directly.

Change-Id: I7514736ce922f632f1f737bc496f6783389a42b6
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/cmakeprojectmanager/fileapidataextractor.cpp

@@ -264,7 +264,7 @@ QList<CMakeBuildTarget> generateBuildTargets(const PreprocessedData &input,
                             continue;
 
                         const FilePath buildDir = haveLibrariesRelativeToBuildDirectory ? buildDirectory : currentBuildDir;
-                        FilePath tmp = buildDir.resolvePath(FilePath::fromUserInput(part).onDevice(buildDir));
+                        FilePath tmp = buildDir.resolvePath(buildDir.withNewPath(part));
 
                         if (f.role == "libraries")
                             tmp = tmp.parentDir();