RFE: lint - improve detection of required keys

[bgoncalv]

for example: https://gitlab.com/redhat/centos-stream/tests/kernel/kernel-tests/-/blob/443974f498ddbec25af53e2c8f066d2901c5bd3a/tracepoints/operational/main.fmf#L21

it set

 - type: library
    pattern:
      - /cki_lib/libcki.sh

tmt lint would not fail:

$ tmt lint tracepoints
/tracepoints/operational
pass C000 fmf node passes schema validation
pass C001 summary key is set and is reasonably long
pass T001 correct keys are used
pass T002 test script is defined
pass T003 directory path is absolute
pass T004 test path '/home/bgoncalv/repos/cki/kernel-tests/tracepoints/operational' does exist
skip T005 legacy relevancy not detected
skip T006 legacy 'coverage' field not detected
skip T007 not a manual test
skip T008 not a manual test
pass T009 all library/file requirements specify type

Lint checks on all
pass G001 no duplicate ids detected

but tmt discover would fail with:

$ tmt run discover
/var/tmp/tmt/run-005

/default/plan
    discover
        how: fmf
        directory: /home/bgoncalv/repos/cki/kernel-tests

plan failed

The exception was caused by 1 earlier exceptions

Cause number 1:

    Need 'url' or 'path' to fetch a beakerlib library.