Remove Windows' Readlink fork

[kolyshkin]

This reverts the part of commit 8100e750d (PR #113) that added the fork of Readlink() for Windows.

At that time the fork was done to work around the bug in golang's implementation of os.Readlink() for Windows.

The above bug was never reported upstream, but fortunately it was independently found, reported [1], and fixed [2]. The fix made its way into go-1.13 (there is no mention of that in release notes, so I checked it in git history).

[1] https://github.com/golang/go/issues/30463 [2] https://go-review.googlesource.com/c/go/+/164201

PS since this requires go 1.13 now, add the appropriate guard so the package won't compile on earlier golang

[kolyshkin]

@crosbymichael @estesp @dmcgowan PTAL

[kolyshkin]

@cpuguy83 PTAL

[cpuguy83]

I like the change but I'm not sure I like that we are dropping go1.12 support so quickly.

[kolyshkin]

I like the change but I'm not sure I like that we are dropping go1.12 support so quickly.

1. this is windows only
2. go 1.12 is an unsupported release :skull: anyway, since 1.14 is out

so when this change is disruptive, it means someone is using an unsupported golang release

[cpuguy83]

Understood that go1.12 is no longer a supported release from the go team, but we still use this in docker 19.03 (for now).

[kolyshkin]

So this one is blocked by https://github.com/moby/moby/pull/40592

[kolyshkin]

we still use this in docker 19.03 (for now).

@cpuguy83 we just stopped doing it :) PTAL

[crosbymichael]

LGTM