fix: update tar traversal to respect current director entry

[spiffcs]

Summary

Users of syft reported upstream that they are now seeing an error for OCI scanning when stereoscope hits a tar entry with . as the current directory.

This PR adds a case for making sure the destination path is not the current directory entry.

It also adds a specific skip for this type given that we don't want to stat . and perform any additional actions.

The current check will fail with a . entry given the following:

`/tmp`
`/tmp/`

/tmp does not begin with the prefix /tmp/ so stereoscope thinks we're escaping and errors

This also prevents the error from surfacing in this case:
anchore/syft#2678

[github-actions]

Benchmark Test Results

Benchmark results from the latest changes vs base branch

latest: Pulling from library/ubuntu
tar: Option --mtime: Treating date 'UTC 2019-09-16' as 2019-09-16 00:00:00
goos: linux
goarch: amd64
pkg: github.com/anchore/stereoscope/pkg/file
cpu: AMD EPYC 7763 64-Core Processor                
ctr: 
           │ ./.tmp/benchmark-089bb6e.txt │
           │            sec/op            │
TarIndex-2                   36.21µ ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-089bb6e.txt │
           │             B/op             │
TarIndex-2                  5.560Ki ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-089bb6e.txt │
           │          allocs/op           │
TarIndex-2                    93.00 ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

pkg: github.com/anchore/stereoscope/test/integration
                                      │ ./.tmp/benchmark-089bb6e.txt │
                                      │            sec/op            │
SimpleImage_GetImage/docker-archive-2                   1.246m ± ∞ ¹
SimpleImage_GetImage/podman-2                           17.50m ± ∞ ¹
geomean                                                 4.669m
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-089bb6e.txt │
                                      │             B/op             │
SimpleImage_GetImage/docker-archive-2                  326.0Ki ± ∞ ¹
SimpleImage_GetImage/podman-2                          441.6Ki ± ∞ ¹
geomean                                                379.4Ki
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-089bb6e.txt │
                                      │          allocs/op           │
SimpleImage_GetImage/docker-archive-2                   2.609k ± ∞ ¹
SimpleImage_GetImage/podman-2                           2.829k ± ∞ ¹
geomean                                                 2.717k
¹ need >= 6 samples for confidence interval at level 0.95

ctr: failed to dial "/run/containerd/containerd.sock": connection error: desc = "transport: error while dialing: dial unix /run/containerd/containerd.sock: connect: permission denied"
                                                   │ ./.tmp/benchmark-089bb6e.txt │
                                                   │            sec/op            │
SimpleImage_FetchSquashedContents/docker-archive-2                   21.91µ ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                           21.33µ ± ∞ ¹
geomean                                                              21.62µ
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-089bb6e.txt │
                                                   │             B/op             │
SimpleImage_FetchSquashedContents/docker-archive-2                  2.648Ki ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                          2.648Ki ± ∞ ¹
geomean                                                             2.648Ki
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-089bb6e.txt │
                                                   │          allocs/op           │
SimpleImage_FetchSquashedContents/docker-archive-2                    21.00 ± ∞ ¹
SimpleImage_FetchSquashedContents/podman-2                            21.00 ± ∞ ¹
geomean                                                               21.00
¹ need >= 6 samples for confidence interval at level 0.95