snap: v0: build fixes & updates (#3999)

* snap: drop sudo

Fixes #3989

* snap: ci: fix builds

* snap: ci: developer notifications

* snap: minimise size

* debug commit

* Revert "snap: minimise size"

This reverts commit 77b41a96f68b2c8df833803d0a644172b7db4779.

* fix image info

* Revert "debug commit"

This reverts commit 232baf0ad78335b0f3806456a3f20b82cdf0f24c.

* snap: v1 channels

Related: #3872

* snap: v0 channels

Related #3990
Related #3989

* snap: auto-version (major) detect

* ci: fetch --tags explanation & error checking

* snap: update warning framework

* hardocde warning

* add deprecation dev comment

* remove warning on explicit v0 channel

* tidy comment

* fix py2

scripts/remove_update_warning.patch

@@ -0,0 +1,51 @@
+diff --git a/dvc/updater.py b/dvc/updater.py
+index d6dc372..cfa416f 100644
+--- a/dvc/updater.py
++++ b/dvc/updater.py
+@@ -48,15 +48,7 @@ class Updater(object):  # pragma: no cover
+             logger.debug(msg.format(self.lock.lockfile, action))
+ 
+     def check(self):
+-        if os.getenv("CI") or env2bool("DVC_TEST"):
+-            return
+-
+-        if PKG == "snap":
+-            # hardcoded transition message
+-            version_major = version.parse(self.current).major
+-            self.current = "v{}".format(version_major)
+-            self.latest = "v{}".format(version_major + 1)
+-            self._notify()
++        if os.getenv("CI") or env2bool("DVC_TEST") or PKG == "snap":
+             return
+ 
+         self._with_lock(self._check, "checking")
+@@ -146,17 +138,6 @@ class Updater(object):  # pragma: no cover
+             ),
+             "conda": "Run `{yellow}conda{reset} update dvc`",
+             "choco": "Run `{yellow}choco{reset} upgrade dvc`",
+-            "snap": (
+-                "To upgrade to the latest major release,\n"
+-                "run `{yellow}snap{reset} refresh --channel=latest/beta`, or\n"
+-                "to stay on the current major release track,\n"
+-                "run `{yellow}snap{reset} refresh --channel={current}/stable`"
+-                "\n\n"
+-                "{red}WARNING{reset}: ignoring this message will result in\n"
+-                "snap automatically performing an upgrade soon.\n"
+-                "More information can be found at\n"
+-                "{blue}https://github.com/iterative/dvc/issues/3872{reset}"
+-            ),
+             None: (
+                 "Find the latest release at\n"
+                 "{blue}https://github.com/iterative/dvc/releases/latest{reset}"
+diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
+index 75d0f7d..996db8c 100644
+--- a/snap/snapcraft.yaml
++++ b/snap/snapcraft.yaml
+@@ -30,7 +30,6 @@ parts:
+     override-pull: |
+         snapcraftctl pull
+         snapcraftctl set-version $(cd $SNAPCRAFT_PART_SRC && git describe --tags)
+-        git diff --quiet || error_dirty_build
+         echo 'PKG = "snap"' > $SNAPCRAFT_PART_SRC/dvc/utils/build.py
+         # install all optional extras
+         sed -ri 's/(=install_requires)/\1+all_remotes+hdfs/' $SNAPCRAFT_PART_SRC/setup.py