BUG/MINOR: pattern: Fix pattern lookup for map with opt@ prefix

BUG/MINOR: pattern: Fix pattern lookup for map with opt@ prefix

When we look for a map file reference, the file@ prefix is removed because
if may be omitted. The same is true with opt@ prefix. However this case was
not properly performed in pat_ref_lookup(). Let's do so.

This patch must be backported as far as 3.0.

(cherry picked from commit 7aa9f5ec98436182b2b5ebeb57828d2daf27f20a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 0d07528f558509ee7b3d097e93842de2d6b273e1)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 6da9abc77d9f77a438856d13c1db15c5dd86b5d8)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>