vmimage.subr: Don't log missing files from pkgs

When installing "extra" packages (aka those built from the ports tree),
we record everything being installed in METALOG.pkg; the contents of
that file is appended to METALOG before we generate the filesystem.

There are two cases when files recorded in METALOG.pkg will no longer
exist by the time we create the final disk image:
1. If a pkg bug results in false dependencies being installed which
are later removed by "pkg autoremove", and
2. If the pkg we build and install from /usr/ports is older than the
pkg on pkg.freebsd.org, and pkg gets upgraded automatically as part of
installing extra packages.

The ultimate issue in both cases is that there's no mechanism for
removing entries from METALOG when we run 'pkg delete'.

Address this build breakage by checking, line by line, if filesystem
objects mentioned in METALOG.pkg exist before appending them to METALOG.

Fixes:	6a13aeac3c ("vmimage.subr: pkg autoremove after pkg install")
MFC after:	immediately (needed for 15.0-RC1)

release/tools/vmimage.subr

@@ -292,8 +292,15 @@ vm_extra_pkg_rmcache() {
 buildfs() {
 	local md tmppool
 
+	# Copy entries from METALOG.pkg into METALOG, but first check to
+	# make sure that filesystem objects still exist; some things may
+	# have been logged which no longer exist if a package was removed.
 	if [ -f ${DESTDIR}/METALOG.pkg ]; then
-		cat ${DESTDIR}/METALOG.pkg >> ${DESTDIR}/METALOG
+		while read F REST; do
+			if [ -e ${DESTDIR}/${F} ]; then
+				echo "${F} ${REST}" >> ${DESTDIR}/METALOG
+			fi
+		done < ${DESTDIR}/METALOG.pkg
 	fi
 
 	if [ -n "${NO_ROOT}" ]; then