Implement get_build_targets_impl in GeneralTestsOptimizer

Implement functionality in GeneralTestsOptimizer to find what targets to
build.

This logic is fairly complex and involves checking for test
configs that download general-tests.zip. Then the configs are checked to
see if they're proper test mapping tests (if they use the
'test-mapping-test-group' option). If they are, then TEST_MAPPING
modules are scanned to see if the list of changed files would cause any
test mapping modules to run. The tests are then further filtered by
test-mapping-test-groups used in the test configs.

In case that a test uses general-tests.zip but does not specify
'test-mapping-test-group' then all bets are off and general-tests.zip
is built in its entirety.

package_outputs is still unimplemented so this will need to be
implemented before the optimization can be enabled.

Test: atest build_test_suites_test && atest optimized_targets_test
Bug: 358215235
Change-Id: I6a7eebfd1b06b380799292eb2019ac17c9af5367