analyze_bcpf: Explain why a package is split/single

Previously, the script would determine whether a package was split,
single or could be used as a prefix but did not explain why. This
change provides additional information to explain why they are split.

Bug: 202154151
Test: m analyze_bcpf && analyze_bcpf --bcpf art-bootclasspath-fragment
      m analyze_bcpf && analyze_bcpf --bcpf art-bootclasspath-fragment --fix
      atest --host analyze_bcpf_test
Change-Id: I3e2d5c0b54b5cc028013ce5ea979ebd9b9bf2c0d