meson: Refactor DocBook detection logic to stop when found #322

Job	Run time
Build Netatalk testsuite container	54s
Build Netatalk container	50s
Netatalk container dry run	6s
AFP spec test (Readonly) - AFP 3.4	7s
AFP login test - AFP 3.4	6s
AFP spec test (Readonly) - AFP 2.1	5s
AFP login test - AFP 2.1	8s
AFP spec test - AFP 2.1	59s
AFP spec test - AFP 2.2	56s
AFP spec test - AFP 3.0	1m 4s
AFP spec test - AFP 3.1	1m 5s
AFP spec test - AFP 3.2	1m 5s
AFP spec test - AFP 3.3	1m 4s
AFP spec test - AFP 3.4	1m 6s
	9m 35s

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