Replace symbol traversal with tree traversal when finding top level experimentals #32917

Job	Run time
test	55m 16s
test_windows_fast	36m 14s
MiMa	10m 56s
test_non_bootstrapped	0s
test_scala2_library_tasty	0s
community_build_a	56m 49s
community_build_b	1h 22m 35s
community_build_c	1h 3m 4s
test_sbt	20m 52s
test_windows_full	0s
build-msi-package	0s
test_java8	0s
build-sdk-package / build	7m 51s
build-chocolatey-package / build	21s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 33m 58s

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