cmDepends: all members accept std::string arguments

Most `const char*` arguments converted to `const std::string&`
in `cmDepends` and derived classes.
In addition performed minor code cleanup.

Source/cmLocalUnixMakefileGenerator3.h

@@ -245,7 +245,7 @@ protected:
 
   // Helper methods for dependency updates.
   bool ScanDependencies(
-    const char* targetDir,
+    const std::string& targetDir,
     std::map<std::string, cmDepends::DependencyVector>& validDeps);
   void CheckMultipleOutputs(bool verbose);