Finish target paths

[Vladimir Prus]

This includes two parts. 

First, it is required that after building every main target, the
project target that cause the build compare free properties 
for all virtual targets with free properties in its requirements.
If they are not equal,  "main-target-<name>" should be added
to the subvariant identifier.

Second, it is needed to devise the replacement for 
ALL_LOCATE_TARGET and change virtual-target.actualize
to set location appropriately.