snap: fix/update snap builds

* rename to nvim
* bump base to core18
* fix versioning, match binary
* set prefix to /usr

snap/snapcraft.yaml

@@ -1,5 +1,6 @@
-name: neovim
-version: git
+name: nvim
+base: core18
+adopt-info: nvim
 summary: Vim-fork focused on extensibility and agility.
 description: |
  Neovim is a project that seeks to aggressively refactor Vim in order to:
@@ -9,22 +10,33 @@ description: |
  Enable the implementation of new/modern user interfaces without any modifications to the core source
  Improve extensibility with a new plugin architecture
  For lots more details, see the wiki!
+
+grade: stable  # must be 'stable' to release into candidate/stable channels
 confinement: classic
 
 apps:
-    neovim:
-        command: usr/local/bin/nvim
-        plugs: [network, network-bind, x11]
+    nvim:
+        command: usr/bin/nvim
         environment:
             HOME: /home/$USER
-            VIM: $SNAP/usr/local/share/nvim/runtime
+            VIM: $SNAP/usr/share/nvim/runtime
 
 parts:
-    neovim:
+    nvim:
         source: .
+        override-pull: |
+          snapcraftctl pull
+          major="$(awk '/NVIM_VERSION_MAJOR/{gsub(")","",$2); print $2}' CMakeLists.txt)"
+          minor="$(awk '/NVIM_VERSION_MINOR/{gsub(")","",$2); print $2}' CMakeLists.txt)"
+          patch="$(awk '/NVIM_VERSION_PATCH/{gsub(")","",$2); print $2}' CMakeLists.txt)"
+          version_prefix="v$major.$minor.$patch"
+          git_described="$(git describe --first-parent --dirty 2> /dev/null | perl -lpe 's/v\d.\d.\d-//g')"
+          git_described="${git_described:-$(git describe --first-parent --tags --always --dirty)}"
+          snapcraftctl set-version "${version_prefix}-${git_described}"
         plugin: make
         make-parameters:
             - CMAKE_BUILD_TYPE=Release
+            - CMAKE_INSTALL_PREFIX=/usr
         build-packages:
             - ninja-build
             - libtool
@@ -33,9 +45,10 @@ parts:
             - automake
             - cmake
             - g++
+            - git
+            - gettext
             - pkg-config
             - unzip
-        snap:
-            - usr/local/bin
-            - usr/local/share/nvim
-            - -usr/local/share/man
+        prime:
+            - -usr/share/man
+