cmLocalGenerator: change ImportedGeneratorTargets from vector to map

For large number of targets significant amount of time is spent in cmLocalGenerator::FindGeneratorTargetToUse, which uses find_if on a vector to locate the given target. Using a map instead of vector for ImportedGeneratorTargets provides a significant speedup. In our project with 16k targets I measured a factor 2 speedup in the Generate stage (from 7 to 3.5 minutes).

In addition the unused public GetImportedGeneratorTargets method has been removed.