compliation failure of minimal latex documents

[Karl Berry]

1) I moved l3backend and l3kernel (but not the other l3*) to
collection-latex from collection-latexrecommended.

2) I also made them both "triggers" for format rebuilding for all latex
formats. Previously only l3kernel was. (I suspect that it is how tljack
ended up with his scenario of only l3kernel installed.)

At present, it seems no files from l3backend are read at .fmt time, but
maybe that will change tonight. In any case, clearly it also needs to be
installed if l3kernel is, so fine.  --thanks, karl.