import: crate: Comment out missing dependencies.

* guix/import/crate.scm (package-names->package-inputs): Emit comments.
(make-crate-sexp): Make input into comment if missing.
(crate->guix-package): Take #:mark-missing? argument.
[dependency-name+missing+version+yanked]: Mark as missing.  Rename from
dependency-name+version+yanked.
[sort-map-dependencies]: Adjust.
[remove-missing+yanked-info]: Remove missing info.  Rename from
remove-yanked-info.
* guix/scripts/import/crate.scm (show-help): Explain --mark-missing.
(%options): Add mark-missing option.
(guix-import-crate): Pass mark-missing option as #:mark-missing?.
* doc/guix.texi (Invoking guix import): Document --mark-missing.
* tests/crate.scm ("crate->guix-package-marks-missing-packages"): Add
test.

Change-Id: I065d394e1c04fdc332b8f7f8b9fcbd87c14c6512
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -124,7 +124,7 @@ Copyright @copyright{} 2023 Thomas Ieong@*
 Copyright @copyright{} 2023 Saku Laesvuori@*
 Copyright @copyright{} 2023 Graham James Addis@*
 Copyright @copyright{} 2023, 2024 Tomas Volf@*
-Copyright @copyright{} 2024 Herman Rimm@*
+Copyright @copyright{} 2024, 2025 Herman Rimm@*
 Copyright @copyright{} 2024 Matthew Trzcinski@*
 Copyright @copyright{} 2024 Richard Sent@*
 Copyright @copyright{} 2024 Dariqq@*
@@ -14687,6 +14687,10 @@ imported as well.
 @item --allow-yanked
 If no non-yanked version of a crate is available, use the latest yanked
 version instead instead of aborting.
+@item --mark-missing
+If a crate dependency is not (yet) packaged, make the corresponding
+input in @code{#:cargo-inputs} or @code{#:cargo-development-inputs} into
+a comment.
 @end table
 
 @item elm