[tex-live] Latexmk doesn't work with the newest TL 2018

[Karl Berry]

    Because the wrapper latexmk.exe first finds a wrong script:
    texmf-dist/scripts/make4ht/extensions/latexmk.lua.

I asked Michal to rename it.

However, in principle our wrapper really should prefer,
for a command named "latexmk",
.../texmf-dist/scripts/latexmk/latexmk.pl
to .../texmf-dist/scripts/make4ht/extensions/latexmk.lua,
since the former is in the "right" directory.

(This is why kpsewhich --subdir was invented (at Heiko's suggestion).) k-