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/libs/utils/filepath.h

@@ -163,8 +163,8 @@ public:
     [[nodiscard]] FilePath searchInDirectories(const FilePaths &dirs,
                                                const FilePathPredicate &filter = {}) const;
     [[nodiscard]] Environment deviceEnvironment() const;
-    [[nodiscard]] FilePath onDevice(const FilePath &deviceTemplate) const;
     [[nodiscard]] FilePath withNewPath(const QString &newPath) const;
+    [[nodiscard]] FilePath withNewMappedPath(const FilePath &newPath) const;
 
     using IterateDirCallback
         = std::variant<