P2.0 first cut took the first-alphabetical lib providing a header and then
checked manifest membership. When several installed libs ship the same header
(e.g. DHT118266, DHT_sensor_library, servodht11 all have DHT.h), the stray
first-match got rejected and the header was dropped even though the declared
lib provides it.
_find_manifest_library_for_header: when a manifest is supplied, pick the first
DECLARED library that provides the header. This both selects the right lib and
excludes undeclared ones. No manifest = legacy first-match.
Test strengthened with a stray same-header lib that sorts first.
1d643797b4