inspector: add Target.getTargets and extract TargetManager #4789

Job	Run time
Build slim tarball	1m 58s
aarch64-darwin: with shared libraries	1h 38m 37s
x86_64-linux: with shared libraries	1h 49m 58s
aarch64-linux: with shared libraries	1h 16m 37s
x86_64-darwin: with shared libraries	2h 40m 51s
	7h 28m 1s

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