snap: declare the dual license and ship the license texts

The store listing carries the crate's MIT OR GPL-2.0-only
expression, and both texts ride along in the snap under
/usr/share/doc/pkh: the MIT grant requires the notice to
accompany copies, and GPL-2 requires the license text with
distribution.

snap/snapcraft.yaml

@@ -9,6 +9,7 @@ description: |
   This snap uses classic confinement and carries the packaging
   toolchain it drives (dpkg-dev, git, mmdebstrap, lintian, quilt, ...)
   so it behaves the same on any Debian/Ubuntu host.
+license: MIT OR GPL-2.0-only
 adopt-info: pkh-part
 
 confinement: classic
@@ -83,6 +84,12 @@ parts:
     override-prime: |
       craftctl default
       ln -sfn fakeroot-sysv "${CRAFT_PRIME}/usr/bin/fakeroot"
+      # Ship the license texts with the binary: the MIT grant requires
+      # the notice to accompany copies, and GPL-2 requires the license
+      # text alongside distribution.
+      mkdir -p "${CRAFT_PRIME}/usr/share/doc/pkh"
+      cp "${CRAFT_PART_SRC}/LICENSE-MIT" "${CRAFT_PART_SRC}/LICENSE-GPL" \
+        "${CRAFT_PRIME}/usr/share/doc/pkh/"
       # Classic-confined ELFs default to the host loader, which pins the
       # snap to hosts shipping at least the build environment's glibc,
       # and cannot see the libraries deduplicated against the base.