sidecar files are sought despide the GDAL_DISABLE_READDIR_ON_OPEN

[lucianpls]

GDAL_DISABLE_READDIR_ON_OPEN has two states:
FALSE (default), sidecar files are sought
EMPTY_DIR - Disable looking for sidecar files
However, with MRF, some files are still sought. This takes considerable time in a busy folder or virtual storage (VSIL)

FALSE: 7 files
*.mrf.aux.xml, *.aux, *.AUX, *.mrf.aux, *.mrf.AUX, *.mrf.msk, *.mrf.MSK
EMPTY_DIR, it is still looking for 3 files:
*.mrf.aux.xml, *.mrf.msk, *.mrf.MSK

[lucianpls]

This issue creates unnecessary delays on opening an MRF located on a slow or remote virtual storage.