validate the literal directory "subprojects" when checking sandbox violations
validate the literal directory "subprojects" when checking sandbox violations

We do not want anyone touching this entire directory tree, but due to
the way it was implemented, we only checked if its direct parent was a
subproject violation. This generally worked, unless people tried to add
`subprojects/` as an include directory.

Patch this hole. It now provides the same warning any sandbox violation
does (but is not currently an error, just a "will become an error in the
future").

pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson