Migrate github.com/docker/docker/pkg/symlink

[thaJeztah]

• replaces Merge pkg/truncindex and pkg/symlink from moby/moby #53
• closes Merge pkg/truncindex and pkg/symlink from moby/moby #53
• relates to Copy pkg/symlink and pkg/truncindex from moby/moby containerd/containerd#4631

This migration was taken from https://github.com/moby/moby/commits/0b93c6e1315aae52638474476c9e91f2677d2e52/pkg/symlink

Strategy taken:

# install filter-repo (https://github.com/newren/git-filter-repo/blob/main/INSTALL.md)
brew install git-filter-repo

cd ~/projects

# create a temporary clone of docker
git clone https://github.com/docker/docker.git moby_symlink
cd moby_symlink

# create branch to work with
git checkout -b migrate_pkg_symlink

# remove all code, except for pkg/symlink
git filter-repo --path pkg/symlink

# go to the target github.com/moby/sys repository
cd ~/projects/moby-sys

# create a branch to work with
git checkout -b integrate_moby_symlink

# add the temporary repository as an upstream and make sure it's up-to-date
git remote add moby_symlink ~/projects/moby_symlink
git fetch moby_symlink

# merge the upstream code, rewriting "pkg/symlink" to "symlink"
git merge --allow-unrelated-histories --signoff -S -Xsubtree=pkg/ moby_symlink/migrate_pkg_symlink

After the above;

• symlink: remove "github.com/docker/docker" import enforcement comments
• symlink: add go.mod
• Makefile: run tests on symlink module
• symlink: add package doc

[thaJeztah]

I manually marked the DCO test to pass (see #53 (comment))

[dims]

LGTM 🚢 🚀 !!

[thaJeztah]

When working on this PR, I recalled there's some things (not for this module alone) we need to address around the licensing. I noticed that while the repository has a top-level LICENSE in place, go submodules effectively act as individual repositories, which means that the individual modules don't have a license.

I think we need a LICENSE and NOTICE in each of the submodules, and while we're at it, add license headers to individual files (as is done in containerd/containerd), and probably change copyright to The Moby authors (or something along those lines) going forward.

I'll create a tracking issue for that.

[thaJeztah]

ping @kolyshkin @cpuguy83 PTAL

[cpuguy83]

LGTM

[kolyshkin]

go 1.15 adds (*testing.TB).TempDir() which would be easier to use and won't require a cleanup.

But we'll have to wait until at least go 1.17 to use it :-\

[thaJeztah]

Ah, nice; didn't know about the TempDir(), but we can start using CleanUp(). I did a PR for that in moby/term#21. That repo was still testing against go 1.13 for backward compatibility, but I see this repo only tests go 1.14 and go 1.15, so I can make a similar change here