Replace symbol traversal with tree traversal when finding top level experimentals #32963

Job	Run time
test	55m 17s
test_non_bootstrapped	0s
test_windows_fast	32m 49s
community_build_a	51m 42s
community_build_b	1h 24m 40s
community_build_c	1h 8m 51s
MiMa	11m 29s
test_scala2_library_tasty	0s
test_windows_full	0s
test_java8	0s
test_sbt	19m 44s
build-msi-package	0s
build-sdk-package / build	8m 18s
test-msi-package	0s
build-chocolatey-package / build	19s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 33m 9s

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