>
> I understand that this is your motivation, but could you make it
> more concrete?  E.g. give at least one package manager which does that
> (giving the actual commands which lead to the source being placed there)
> and uses "emacs" for <package-name>.


I will give you the simplest visual example possible: Cygwin (see the
attachment). Sources will end up under "/usr/local/src/emacs".

Thanks.  But the fact is, no GNU project I know of has rules in its
> Makefiles to install sources, to /usr/local/src or anywhere else.


It however turns out that you still already follow the convention by
default. The current default directory structure of Emacs installation is:

bin
libexec
share
var

Is this by accident? I'm quite sure that it is not. Adding "src/emacs" and
supporting it by default is just the right thing to do.

Anyway, I will prepare a patch for consideration soon.