export_library_dependencies: Use original link libraries internally

This command was using the cmTarget::GetLinkLibraries method with a
comment explaining how execution order gives it the dependencies before
analysis.  Just use cmTarget::GetOriginalLinkLibraries instead.
cmTarget: Rename old-style link dependencies code as "ForVS6"

The old link dependency analysis is now needed only for the VS 6
generator code delimited by CM_USE_OLD_VS6 to support project-provided
project templates.  Rename the related cmTarget members to be "ForVS6".
cmTarget: Run old-style link dependencies only for VS 6

Invoke it at runtime only with the VS 6 generator.  No other generators
need it.
cmTarget: Compile old-style link dependencies only for VS 6

Compile all the "ForVS6" cmTarget members only on Windows.  No other
platforms support the VS 6 generator.
Merge topic 'target-legacy-depends-only-for-vs6' into next

8a205b98 cmTarget: Compile old-style link dependencies only for VS 6
d57be904 cmTarget: Run old-style link dependencies only for VS 6
b3b44d13 cmTarget: Rename old-style link dependencies code as "ForVS6"
80cb12bb export_library_dependencies: Use original link libraries internally