Exclude lex sub package name from (caller $n)[3]

Commit ae77754ae caused it to start including the package name.  Pre-
viously, lexical subs were reported as ‘(unknown)’.

Now we have more expected output:

$ ./perl -Ilib -XMfeature=:all -e 'my sub x{warn +(caller 0)[3]} x'
x at -e line 1.