Support file:// mirrors the same way copy:// is supported

As long as you can make it so that the same path to the deb file works
inside and outside of the chroot using file:// as a mirror is no longer
a problem with the previous work.

coverage.sh

@@ -1526,7 +1526,7 @@ else
 	skipped=$((skipped+1))
 fi
 
-print_header "mode=$defaultmode,variant=apt: fail with file:// mirror"
+print_header "mode=$defaultmode,variant=apt: file:// mirror"
 cat << END > shared/test.sh
 #!/bin/sh
 set -eu
@@ -1535,13 +1535,9 @@ if [ ! -e /mmdebstrap-testenv ]; then
 	echo "this test requires the cache directory to be mounted on /mnt and should only be run inside a container" >&2
 	exit 1
 fi
-ret=0
-$CMD --mode=$defaultmode --variant=apt $DEFAULT_DIST /tmp/debian-chroot.tar "deb file:///mnt/cache/debian unstable main" || ret=\$?
+$CMD --mode=$defaultmode --variant=apt $DEFAULT_DIST /tmp/debian-chroot.tar "deb file:///mnt/cache/debian $DEFAULT_DIST main"
+tar -tf /tmp/debian-chroot.tar | sort | diff -u tar1.txt -
 rm /tmp/debian-chroot.tar
-if [ "\$ret" = 0 ]; then
-	echo expected failure but got exit \$ret >&2
-	exit 1
-fi
 END
 if [ "$HAVE_QEMU" = "yes" ]; then
 	./run_qemu.sh

mmdebstrap

@@ -2135,8 +2135,13 @@ sub run_download() {
     # strip the the chroot directory from the filenames
     foreach my $deb (@dl_debs) {
         if (rindex $deb, $options->{root}, 0) {
-            error "package file $deb not in chroot directory"
-              . " -- use copy:// instead of file://";
+            if (-e "$options->{root}/$deb") {
+                push @essential_pkgs, $deb;
+            } else {
+                error "package file $deb not accessible from chroot directory"
+                  . " -- use copy:// instead of file:// or a bind-mount";
+            }
+            next;
         }
         if (-e $deb) {
             push @essential_pkgs, substr($deb, length($options->{root}));
@@ -6952,12 +6957,6 @@ as the non-root user, then as a workaround you could run C<chmod 600
 /etc/dpkg/dpkg.cfg.d/*> so that the config files are only accessible by the
 root user. See Debian bug #808203.
 
-The C<file://> URI type cannot be used to install the essential packages. This
-is because B<mmdebstrap> uses dpkg to install the packages that apt places into
-F</var/cache/apt/archives> but with C<file://> apt will not copy the files even
-with C<--download-only>. Use C<copy://> instead, which is equivalent to
-C<file://> but copies the archives into F</var/cache/apt/archives>.
-
 With apt versions before 2.1.16, setting C<[trusted=yes]> or
 C<Acquire::AllowInsecureRepositories "1"> to allow signed archives without a
 known public key or unsigned archives will fail because of a gpg warning in the