btf: only return exact matches from AnyTypesByName
btf: only return exact matches from AnyTypesByName

Change AnyTypesByName so that essential names stay an implementation
detail of the btf package: we need spec.namedTypes for fast
flavour lookup, but it's not clear that we should expose the flavour
name distinction to the users.

Fix this by only returning types which have an exact name match. This
means that `AnyTypesByName("ambiguous")` does not return ambiguous___flavour.

cilium/ebpf,cilium/ebpf