Fixes <https://bugs.gnu.org/42009>.
Reported by Marinus <marinus.savoritias@disroot.org>.

* gnu/packages.scm (generate-package-cache)[entry-key, entry<?]
[variables]: New variables.
[expand-cache]: Change to take two arguments.
[exp]: Fold over VARIABLES.

* gnu.scm (%try-use-modules): Use 'formatted-message' instead of
'&message'.
* gnu/machine/digital-ocean.scm (maybe-raise-unsupported-configuration-error):
Likewise.
* gnu/machine/ssh.scm (machine-check-file-system-availability): Likewise.
(machine-check-building-for-appropriate-system): Likewise.
(deploy-managed-host): Likewise.
(maybe-raise-unsupported-configuration-error): Likewise.
* gnu/packages.scm (search-patch): Likewise.
* gnu/services.scm (%service-with-default-value): Likewise.
(files->etc-directory): Likewise.
(fold-services): Likewise.
* gnu/system.scm (locale-name->definition*): Likewise.
* gnu/system/mapped-devices.scm (check-device-initrd-modules): Likewise.
(check-luks-device): Likewise.
* guix/channels.scm (latest-channel-instance): Likewise.
* guix/cve.scm (json->cve-items): Likewise.
* guix/git-authenticate.scm (commit-signing-key): Likewise.
(commit-authorized-keys): Likewise.
(authenticate-commit): Likewise.
(verify-introductory-commit): Likewise.
* guix/remote.scm (remote-pipe-for-gexp): Likewise.
* guix/scripts/graph.scm (assert-package): Likewise.
* guix/scripts/offload.scm (private-key-from-file*): Likewise.
* guix/ssh.scm (authenticate-server*): Likewise.
(open-ssh-session): Likewise.
(remote-inferior): Likewise.
* guix/ui.scm (matching-generations): Likewise.
* guix/upstream.scm (package-update): Likewise.
* tests/channels.scm ("latest-channel-instances, missing introduction for 'guix'"):
Catch 'formatted-message?'.
("authenticate-channel, wrong first commit signer"): Likewise.
* tests/lint.scm ("patches: not found"): Adjust message string.
* tests/packages.scm ("patch not found yields a run-time error"): Catch
'formatted-message?'.
* guix/lint.scm (check-patch-file-names): Handle 'formatted-message?'.
(check-derivation): Ditto.

This allows 'tests/packages.scm' to mock it.

* gnu/packages.scm (find-best-packages-by-name): Set! it at the top
level to prevent it from being inline on Guile 3.

In Guile < 2.9.7, autoloading a module would give you access to all its
bindings.  In future versions, autoloading a module gives access only to
the listed bindings, as per #:select (see <https://bugs.gnu.org/38895>).

This commit adjusts autoloads to the new semantics, allowing Guix to be
built with Guile 2.9.7/2.9.8.

* guix/build/download.scm <top level>: Remove call to 'module-autoload!'.
(load-gnutls): New procedure.
(tls-wrap): Call it.
* guix/git.scm <top level>: Remove call to 'module-autoload!'.
(load-git-submodules): New procedure.
(update-submodules): Call it instead of 'resolve-interface'.
* gnu/bootloader/grub.scm: Replace #:autoload with #:use-module.
* gnu/packages.scm: Likewise.
* gnu/packages/ssh.scm: Likewise.
* gnu/packages/tex.scm: Likewise.
* gnu/services/cuirass.scm: Likewise.
* gnu/services/mcron.scm: Likewise.
* guix/lint.scm: Augment list of bindings in #:autoload.
* guix/scripts/build.scm: Likewise.
* guix/scripts/gc.scm: Likewise.
* guix/scripts/pack.scm: Likewise.
* guix/scripts/publish.scm: Likewise.
* guix/scripts/pull.scm: Likewise.
* guix/utils.scm: Remove unnecessary #:autoload clauses; replace one
of them with #:use-module.

Fixes <https://bugs.gnu.org/37748>.
Reported by Marius Bakke <mbakke@fastmail.com>.

This is a followup to d2d63e20d5b981009b61bf416b4d7b516e8f1f34.

* gnu/packages.scm (fold-available-packages): Use 'supported-package?'
instead of (member ... (package-supported-systems ...)) to match what
'generate-package-cache' does.

* gnu/packages.scm (generate-package-cache): Use 'supported-package?'
instead of 'package-supported-systems'.

Until now the caller couldn't tell the different between a spec like
"foo:out" and one like "foo".  This change allows users to distinguish
between these two cases.

* gnu/packages.scm (specification->package+output): Disable output
membership test when OUTPUT = #f and SUB-DRV = #f.
* tests/packages.scm ("specification->package+output")
("specification->package+output invalid output")
("specification->package+output no default output")
("specification->package+output invalid output, no default"): New tests.

* gnu/packages.scm (%bootstrap-binaries-path, search-bootstrap-binary):
Remove.
* gnu/packages/bootstrap.scm (bootstrap-executable): Export.
* guix/tests.scm (bootstrap-binary-file, search-bootstrap-binary):
Export.
* tests/derivations.scm: Remove (gnu packages) import.
* tests/grafts.scm: Likewise.
* tests/guix-daemon.sh: Likewise.