[tex-live] t1code.tex not found

[Karl Berry]

I can't build latex.efmt with the current setup, t1code.tex is not
found.  I was using the master /home/tlprod checkout on tug, so I'm
reasonably sure it's not a p4 issue.

...
  (/home/tlprod/Master/texmf-dist/tex/generic/hyphen/czhyph.tex
  ! I can't find file `t1code.tex'.

The file is present at:
/home/tlprod/Master/texmf-dist/tex/csplain/base/t1code.tex

And indeed, csplain is not being searched, according to
/home/tlprod/Master/texmf/web2c/texmf.cnf, which has:
  TEXINPUTS.tex =			.;$TEXMF/tex/{plain,generic,}//

I do not know what the right fix is.  Or why it is apparently working
for others.

Help?