Further failsafe check for available APT packages

- "apt-cache show package" succeeds as well if package is listed as (optional) dependency or conflict by another package, hence is not a 100% reliable measure.
- There is no command which explicitly checks which package/name can be selected by apt-get for install. An install simulation/dry-run is possible as it was before Pi-hole v5.1, or the whole package cache can be scraped, which is still the less time consuming solution.
- Allow to succeed if another package "provides" it, like "php7.3-apcu" provided by "php-apcu" or "awk" provided by "mawk" and "gawk", in which case the non-virtual package is selected automatically by apt-get.

For reference: 066b89fa41

Signed-off-by: MichaIng <micha@dietpi.com>

automated install/basic-install.sh

@@ -85,6 +85,8 @@ QUERY_LOGGING=true
 INSTALL_WEB_INTERFACE=true
 PRIVACY_LEVEL=0
 CACHE_SIZE=10000
+# Placeholder variable for the list of available APT packages to be parsed subsequently
+APT_PACKAGE_LIST=""
 
 if [ -z "${USER}" ]; then
   USER="$(id -un)"
@@ -179,6 +181,19 @@ is_command() {
     command -v "${check_command}" >/dev/null 2>&1
 }
 
+is_apt_package(){
+    # Checks whether a package, or one that provides it, is available in
+    # the installed APT repository lists.
+    local check_package=$1
+
+    # Obtain the list of available packages once
+    if [[ -z $APT_PACKAGE_LIST ]]; then
+        APT_PACKAGE_LIST=$(apt-cache dumpavail | grep -E '^P(ackage|rovides):')
+    fi
+
+    grep -qE " $check_package(,|$)" <<< "$APT_PACKAGE_LIST"
+}
+
 os_check() {
     if [ "$PIHOLE_SKIP_OS_CHECK" != true ]; then
         # This function gets a list of supported OS versions from a TXT record at versions.pi-hole.net
@@ -303,10 +318,10 @@ if is_command apt-get ; then
     # Update package cache. This is required already here to assure apt-cache calls have package lists available.
     update_package_cache || exit 1
     # Debian 7 doesn't have iproute2 so check if it's available first
-    if apt-cache show iproute2 > /dev/null 2>&1; then
+    if is_apt_package iproute2; then
         iproute_pkg="iproute2"
     # Otherwise, check if iproute is available
-    elif apt-cache show iproute > /dev/null 2>&1; then
+    elif is_apt_package iproute; then
         iproute_pkg="iproute"
     # Else print error and exit
     else
@@ -326,10 +341,10 @@ if is_command apt-get ; then
     # Check if installed php is v 7.0, or newer to determine packages to install
     if [[ "$phpInsNewer" != true ]]; then
         # Prefer the php metapackage if it's there
-        if apt-cache show php > /dev/null 2>&1; then
+        if is_apt_package php; then
             phpVer="php"
         # Else fall back on the php5 package if it's there
-        elif apt-cache show php5 > /dev/null 2>&1; then
+        elif is_apt_package php5; then
             phpVer="php5"
         # Else print error and exit
         else
@@ -341,9 +356,9 @@ if is_command apt-get ; then
         phpVer="php$phpInsMajor.$phpInsMinor"
     fi
     # We also need the correct version for `php-sqlite` (which differs across distros)
-    if apt-cache show "${phpVer}-sqlite3" > /dev/null 2>&1; then
+    if is_apt_package "${phpVer}-sqlite3"; then
         phpSqlite="sqlite3"
-    elif apt-cache show "${phpVer}-sqlite" > /dev/null 2>&1; then
+    elif is_apt_package "${phpVer}-sqlite"; then
         phpSqlite="sqlite"
     else
         printf "  %b Aborting installation: No SQLite PHP module was found in APT repository.\\n" "${CROSS}"