cmInstallGenerator: Refactor computation of absolute install dest

Replace the GetInstallDestination method, which looked up the
Destination ivar, with a new ConvertToAbsoluteDestination method that
takes the nominal destination as an argument.  Update call sites
accordingly.  This will allow some clients to transform the install
destination before calling the method.

Source/cmInstallGenerator.h

@@ -52,7 +52,7 @@ public:
 
   /** Get the install destination as it should appear in the
       installation script.  */
-  std::string GetInstallDestination() const;
+  std::string ConvertToAbsoluteDestination(std::string const& dest) const;
 
   /** Test if this generator installs something for a given configuration.  */
   bool InstallsForConfig(const std::string& config);