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/projectexplorer/project.cpp

@@ -872,6 +872,15 @@ bool Project::hasParsingData() const
     return d->m_hasParsingData;
 }
 
+const ProjectNode *Project::findNodeForBuildKey(const QString &buildKey) const
+{
+    const ProjectNode *result = nullptr;
+    d->m_rootProjectNode->forEachProjectNode([buildKey](const ProjectNode *node) {
+        return node->buildKey() == buildKey;
+    });
+    return result;
+}
+
 ProjectImporter *Project::projectImporter() const
 {
     return nullptr;