Fix go.mod; simplify pkg/mount.Mounted

[kolyshkin]

Carry of #720
Closes: #723

1. go.mod: fix go version

Commit 1a28477 (inadvertently?) changed go version in go.mod from 1.13
to 1.15.

A "go " line in go.mod sets minimum go version required for the
package (see [1]). I don't think containers/storage suddenly requires go
1.15+.

Set it to 1.14, which is currently the oldest supported release.

[1] https://golang.org/ref/mod#go-mod-file-go

2. pkg/mount: rm path normalization from Mounted

Since moby/sys/mountinfo v0.2.0, the path does not have to be
normalized, so drop our wrapper.

Note this slightly change the call semantics: previously, a non-existing
path resulting in an error (wrapped ENOENT), while now it will return
"not mounted". The change is intentional -- the call is not to check if
the path exists, it is to check whether it is mounted.

[rhatdan]

LGTM

[rhatdan]

@nalind @vrothberg @TomSweeneyRedHat @giuseppe @saschagrunert PTAL

[vrothberg]

While I am okay to revert the go version in the module I think we need to agree what it actually means. To my understanding, it does not set a lower bound of which versions of go can compile the code. However, it allows for using new features of go tooling. For instance, go >= 1.14 will default to -mod=vendor if a ./vendor dir is present. Such features seem useful to me.

[vrothberg]

Conditional LGTM :^)

[rhatdan]

LGTM

[kolyshkin]

To my understanding, it does not set a lower bound of which versions of go can compile the code.

AFAIK this is about both tooling features as well as the code (I barely remember seeing some code-related error, alas I don't remember what it was).