Faster tuist generate for projects with large SwiftPM dependencies

Fixed a performance regression that caused tuist generate to stall for a long time when mapping large SwiftPM graphs, such as the AWS SDK for Swift (~800 targets). Tuist no longer spawns a separate swift --version subprocess for each target, and it builds the package-targets lookup once per package instead of repeatedly. In a synthetic 800-module package, wall time dropped from about 14.6s to about 2.6s and system time from about 31s to about 1.1s.