bug#44863: Warning about importing a MELPA package

[Zhu Zihao]

ELPA importer supports MELPA[1] currently. But MELPA is a rolling
archive, which does not persist any old version tarball of
package, and harmful for reproducible build.

It's still useful for Guix packager to import package from MELPA to
draft a sketch of Emacs package, but it's not a reliable download
service. We may better warn user don't submit package which download url
belongs to MELPA. Maybe emit warning while executing `guix import elpa -a
melpa XXX`, or writing this rule to manual.

[1]: the stable archive is "MELPA stable", I use term "MELPA" to refer
to the unstable one.

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Zihao

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