/usr/lib/tcltk/tcLex1.2/pkgIndex.tcl is in tcl-tclex 1.2a1-16.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1
package ifneeded tcLex 1.2 [list load [file join $dir tcLex.so.1.2] tcLex]