Fix internal target lookup performance regression

Refactoring in commit v3.5.0-rc1~272^2~13 (cmGlobalGenerator: Remove
direct storage of targets, 2015-10-25) replaced an efficient data
structure mapping from target name to cmTarget instance with a linear
search.  Lookups through cmGlobalGenerator::FindTarget are done a lot.
Restore the efficient mapping structure with a name indicating its
purpose.

Reported-by: Bartosz Kosiorek <gang65@poczta.onet.pl>