go.mod: change imports to github.com/distribution/distribution/v3

Go 1.13 and up enforce import paths to be versioned if a project
contains a go.mod and has released v2 or up.

The current v2.x branches (and releases) do not yet have a go.mod,
and therefore are still allowed to be imported with a non-versioned
import path (go modules add a `+incompatible` annotation in that case).

However, now that this project has a `go.mod` file, incompatible
import paths will not be accepted by go modules, and attempting
to use code from this repository will fail.

This patch uses `v3` for the import-paths (not `v2`), because changing
import paths itself is a breaking change, which means that  the
next release should increment the "major" version to comply with
SemVer (as go modules dictate).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
       with:
-        path: src/github.com/docker/distribution
+        path: src/github.com/distribution/distribution
         fetch-depth: 50
 
     - name: Set up Go
@@ -39,7 +39,7 @@ jobs:
           cd /tmp && go get -u github.com/vbatts/git-validation
 
     - name: Build
-      working-directory: ./src/github.com/docker/distribution
+      working-directory: ./src/github.com/distribution/distribution
       run: |
         DCO_VERBOSITY=-q script/validate/dco
         GO111MODULE=on script/setup/install-dev-tools
@@ -52,4 +52,4 @@ jobs:
 
     - uses: codecov/codecov-action@v1
       with:
-        directory: ./src/github.com/docker/distribution
+        directory: ./src/github.com/distribution/distribution