meson: subprojects: replace submodules with wrap files

Compared to submodules, .wrap files have several advantages:

* option parsing and downloading is delegated to meson

* the commit is stored in a text file instead of a magic entry in the
  git tree object

* we could stop shipping external dependencies that are only used as a
  fallback, but not break compilation on platforms that lack them.
  For example it may make sense to download dtc at build time, controlled
  by --enable-download, even when building from a tarball.  Right now,
  this patch does the opposite: make-release treats dtc like libvfio-user
  (which is not stable API and therefore hasn't found its way into any
  distros) and keycodemap (which is a copylib, for better or worse).

dependency() can fall back to a wrap automatically.  However, this
is only possible for libraries that come with a .pc file, and this
is not very common for libfdt even though the upstream project in
principle provides it; it also removes the control that we provide with
--enable-fdt={system,internal}.  Therefore, the logic to pick system
vs. internal libfdt is left untouched.

--enable-fdt=git is removed; it was already a synonym for
--enable-fdt=internal.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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