buildtoc - fix inner links when generating perltoc.pod

[haarg]

When generating perltoc.pod, the headings taken from all of the various
pod documents included in core. These headings can have arbitrary
content, including links. A link with a second but without a name won't
work once it's moved into the perltoc.pod file, so convert any links
like this to include the name of the pod document they are being taken
from.

[haarg]

This is meant to fix #16595, which I believe was incorrectly closed with a change filed on Test-Harness. The Pod in TAP::Harness is completely valid, it just gets broken when partially copied to a different file. The responsibility to deal with that lies with core, not Test-Harness.

[jkeenan]

I checked out this branch, configured and called make test_porting, but then got this failure:

$ cd t; ./perl harness porting/exec-bit.t; cd -
porting/exec-bit.t .. 1/? # Failed test 118 - tarball will chmod +x ../pod/buildtoc at porting/exec-bit.t line 60
# Remove the exec bit or add '../pod/buildtoc' to Porting/exec-bit.txt
porting/exec-bit.t .. Failed 1/135 subtests 

Test Summary Report
-------------------
porting/exec-bit.t (Wstat: 0 Tests: 135 Failed: 1)
  Failed test:  118
Files=1, Tests=135,  0 wallclock secs ( 0.03 usr  0.01 sys +  0.07 cusr  0.03 csys =  0.14 CPU)
Result: FAIL

This p.r. does, however clear up the two ERRORs reported in #16595 when one calls podchecker pod/perltoc.pod.

[jkeenan]

Most recent commit clears up porting test problem. +1 to merge. Thanks.