mkosi: Copy sources under /usr in the image

Originally, the source code was copied under /root/src.
This home directory is part of root FS and the new mkosi building
paradigm has only ephemeral root FS that is generated lazily.
Any files placed on the root FS in the build environment are that
excluded from the final image.

It is useful to have source codes available in the image's runtime (not
build time) environment for debugging.

ExtraTrees= as used currently are ineffective, so change the destination
to copy files under /usr to achieve the intention.

gdb sees source files as:
> 1354	../src/src/systemctl/systemctl.c: No such file or directory.

Modify gdb configration in the built image accordingly (that file cannot
be in /root neither) to resolve to the moved sources.

(Commit fdecbf7 ("Enable unprivileged image builds") envisions bind
mounting or virtiofsd for nspawn or qemu containers respectively.)

mkosi.presets/system/mkosi.conf

@@ -8,7 +8,7 @@ Dependencies=base
 Autologin=yes
 BaseTrees=../../mkosi.output/base
 ExtraTrees=../../mkosi.output/base-systemd
-ExtraTrees=../../src:/root/src
+ExtraTrees=../../src:/usr/src/src
 Initrds=../../mkosi.output/initrd
 Packages=
         acl

mkosi.presets/system/mkosi.extra/usr/share/factory/mkosi/gdbinit.d/systemd.gdb

@@ -1,3 +1,3 @@
 set debuginfod enabled off
 set build-id-verbose 0
-set substitute-path ../src /root/src
+set substitute-path ../src /usr/src