ProjectExplorer: Reduce qmake specific logic

- add a QString FileNode::buildKey(), returning the build key
  for a build represented by that node, if any.

     * for QmakeProFileNodes the build key is traditionally equal
       to the file name, so use that.

     * for QbsProductNode use the uniqueProductName()

- add a Project::findNodeForBuildKey(QString buildKey) convenience
  function searching a Project(!)Node matching that build key.
  That's the only use case there is right now, and I see no reason
  yet to travers all files (yet).

Change-Id: I388c0e06c03111e12d630899d762448e974a5737
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qmakeprojectmanager/qmakenodes.cpp

@@ -223,6 +223,11 @@ bool QmakeProFileNode::showInSimpleTree() const
     return showInSimpleTree(projectType()) || m_project->rootProjectNode() == this;
 }
 
+QString QmakeProFileNode::buildKey() const
+{
+    return filePath().toString();
+}
+
 QmakeProFile *QmakeProFileNode::proFile() const
 {
     return static_cast<QmakeProFile*>(QmakePriFileNode::priFile());