feat(info): show the install reason and drop the indentation

N? and `info PKG` now print three lines, none indented:

    PKG: <description>
    reason: explicitly installed | installed as a dependency | unknown | not installed
    required by: <installed dependents, comma separated> | nothing

The reason comes from a new <be>_reason backend function: apt-mark
showauto / showmanual (dpkg decides for a package in neither list), the
Install Reason of pacman -Qi, dnf repoquery --installed %{reason} (dnf4
and dnf5 words) and the Installed line of zypper info (rpm plus libzypp's
AutoInstalled when zypper is locked). A row whose old version is "-" is
not installed and the backend is not asked. The "(installed)" qualifier
moves to --help, README and DESIGN.

The rpm E2E fixture and e2e_reset now also mark su-dummy-a and su-dummy-c
as user-installed on dnf, which records plain rpm installs as "unknown",
so 1? and 2? show both reasons on all 12 distros. The verified commands
are recorded in discovery/{apt,pacman,rpm}.md.

tests/e2e/skeptical-updater.bats

@@ -32,14 +32,27 @@ zypper_verb() { case $(. /etc/os-release && printf '%s' "$ID") in opensuse-tumbl
 	refute_line --regexp '^.\[4\]'
 }
 
-@test "N? prints the description; su-dummy-b is required by su-dummy-a" {
+@test "N? prints the description, the install reason and who requires it" {
+	local -a lines_=()
+	local i a=-1 b=-1
 	run skeptical-updater --no-refresh <<<$'1?\n2?\nq'
 	assert_success
 	assert_line --regexp '^su-dummy-a: .*([Dd]ummy|skeptical-updater).*$'
 	assert_line --regexp '^su-dummy-b: .*([Dd]ummy|skeptical-updater).*$'
-	# [1] su-dummy-a has no installed reverse dependency, [2] su-dummy-b is needed by su-dummy-a
-	assert_line '  required by (installed): nothing'
-	assert_line '  required by (installed): su-dummy-a'
+	# [1] su-dummy-a was installed explicitly and nothing needs it; [2] su-dummy-b was installed as
+	# a dependency of su-dummy-a - the two lines right under each description, no indentation
+	mapfile -t lines_ <<<"$output"
+	for i in "${!lines_[@]}"; do
+		case ${lines_[i]} in
+		'su-dummy-a: '*) a=$i ;;
+		'su-dummy-b: '*) b=$i ;;
+		esac
+	done
+	[[ ${lines_[a + 1]} == 'reason: explicitly installed' && ${lines_[a + 2]} == 'required by: nothing' ]] ||
+		fail "1? (su-dummy-a): got '${lines_[a + 1]}' / '${lines_[a + 2]}'"
+	[[ ${lines_[b + 1]} == 'reason: installed as a dependency' && ${lines_[b + 2]} == 'required by: su-dummy-a' ]] ||
+		fail "2? (su-dummy-b): got '${lines_[b + 1]}' / '${lines_[b + 2]}'"
+	refute_output --partial 'required by (installed)'
 	assert_line 'Quit: nothing was upgraded.'
 }
 
@@ -250,7 +263,11 @@ zypper_verb() { case $(. /etc/os-release && printf '%s' "$ID") in opensuse-tumbl
 	assert_line --partial "$BE"$'\t'"su-dummy-c"$'\t'"user"
 	run asuser skeptical-updater info su-dummy-b
 	assert_success
-	assert_line '  required by (installed): su-dummy-a'
+	assert_line 'reason: installed as a dependency'
+	assert_line 'required by: su-dummy-a'
+	run asuser skeptical-updater info su-dummy-a
+	assert_success
+	assert_line 'reason: explicitly installed'
 	run asuser skeptical-updater --dry-run --no-refresh <<<''
 	assert_success
 	assert_line --regexp "$(native_upgrade_regex)"