Remove the return type of the merge Method and modify the existinf spec by reference.

[marcelthole]

This will reduce the time for loading over 1500 Spec files from 2 minutes to 1 second.

new - fixtures contains 1500 files

/var/www # time php bin/openapi-merge fixtures/api.openapi.json --resolve-references=0 -- $(find fixtures/ -type f -iname '*.openapi.json' | xargs) > /dev/null
real    0m 0.86s
user    0m 0.67s
sys     0m 0.18s

old - fixtures contains 1500 files

/var/www # time php bin/openapi-merge fixtures/api.openapi.json --resolve-references=0 -- $(find fixtures/ -type f -iname '*.openapi.json' | xargs) > /dev/null
real    2m 2.03s
user    2m 0.82s
sys     0m 1.16s