CMakePM: Add imported targets to code completion / navigation

This allows e.g. Qt6::Core to be specified as argument for
target_link_libraries and navigation via F2.

Also fixes a bug introduced with
695952f84b which removed the project
specific features due to project being nullptr.

Change-Id: I5efa28c498d337e6bab564533a5445e6c364b26b
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildsystem.h

@@ -121,6 +121,7 @@ public:
 
     const QHash<QString, Utils::Link> &cmakeSymbolsHash() const { return m_cmakeSymbolsHash; }
     CMakeKeywords projectKeywords() const { return m_projectKeywords; }
+    QStringList projectImportedTargets() const { return m_projectImportedTargets; }
 
 signals:
     void configurationCleared();
@@ -227,6 +228,7 @@ private:
     QSet<CMakeFileInfo> m_cmakeFiles;
     QHash<QString, Utils::Link> m_cmakeSymbolsHash;
     CMakeKeywords m_projectKeywords;
+    QStringList m_projectImportedTargets;
 
     QHash<QString, ProjectFileArgumentPosition> m_filesToBeRenamed;