[bug#49181] Fix missing phases in Emacs builds

[Carlo Zancanaro]

Hi there!

I went to install emacs-protobuf-mode today and found that the 
build wasn't working. I investigated and it looks like someone 
renamed the add-source-to-load-path phase to expand-load-path, but 
they left a few references behind. The first of my patches fixes 
that.

The second of my patches is more controversial. It changes 
modify-phases to error out if the asked-for phase doesn't exist in 
add-before/add-after clauses. I think this is the right move, 
because it's hard to imagine when the default behaviour of "add to 
the end of the phases list" is helpful. In most cases the extra 
phases are setup/transformation phases that we need to run before 
the final "install" phase, so it's far more useful to fail early 
than to add these to the end of the phases list.

Carlo

0001-gnu-Fix-references-to-emacs-build-system-s-expand-lo.patch
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0002-guix-Make-modify-phases-error-when-adding-before-aft.patch
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