Dotty

Replace symbol traversal with tree traversal when finding top level experimentals #32977

Job	Run time
test	52m 12s
test_non_bootstrapped	0s
test_scala2_library_tasty	0s
test_windows_fast	31m 33s
MiMa	13m 5s
community_build_a	53m 47s
test_windows_full	0s
community_build_b	1h 24m 9s
community_build_c	1h 1m 34s
test_sbt	20m 19s
build-msi-package	0s
test_java8	0s
build-sdk-package / build	7m 41s
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 24m 39s

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