explTF and expstar destinations do not exist

[blefloch]

When documenting any conditional or expandable function, a hyperlink is made from TF or from a star to a non-existing destination (only existing when expl3 is typeset). This leads to

pdfTeX warning (dest): name{explTF} has been referenced but does not exist, replaced by a fixed one
pdfTeX warning (dest): name{expstar} has been referenced but does not exist, replaced by a fixed one

Presumably fixing that requires keeping track in the aux file whether the destinations are defined anywhere. I have some recollection that there is a package doing precisely that (maybe by Heiko?).

[wspr]

I couldn't find an Oberdiek package to do this unfortunately. Two thoughts here:

1. Patch \hypertarget to write data to the aux file. Not a bad approach, really — when cross-referencing I like being able to detect if a LaTeX reference exists, and doing something similar with hyperref would be natural.

2. Or provide special \CodedocHypertarget commands just for this purpose, which does a more lightweight version of setting a flag whether to hyperlink these symbols or not.

Right now I'm leaning towards option 1. Regardless, maybe time to revive the auxdata work I was doing some time back.

[blefloch]

Fixed by the same commit as #477 since they were actually duplicates.