Adds support for Symlinks in all Tar decompressors

[prologic]

This is useful when using Nomad's Artefact mechanisms which internally
uses hashicorp/go-getter to unpack Tar archives that are built from
container root file systems (which usually contain symlinks for runtime
libraries).

Fixes #60 and supercedes the very old PR #37

(We use a variant of this patch in our production systems and we need to
get this into upstream as soon as conveniently possible)

NB: This uses the
securejoin library to
prevent path escaping and other possible security vulnerabilities that
come with concatenating paths.

[prologic]

This is a redo of #171 that was reverted due to security issues.

[prologic]

Fixed merge conflicts

[prologic]

Please note: There are some failing tests in master that don't appear to have anything to do with my change so CI is failing on a couple of tests.

prologic@Jamess-MacBook
Mon Jul 08 17:05:40
~/Contributions/go-getter
 (fix_symlink_support) 0
$ go test .
--- FAIL: TestGCSGetter (0.00s)
    get_gcs_test.go:40: err: dialing: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
--- FAIL: TestGCSGetter_subdir (0.00s)
    get_gcs_test.go:60: err: dialing: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
--- FAIL: TestGCSGetter_GetFile (0.00s)
    get_gcs_test.go:81: err: dialing: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
--- FAIL: TestGCSGetter_ClientMode_dir (0.00s)
    get_gcs_test.go:113: err: dialing: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
--- FAIL: TestGCSGetter_ClientMode_file (0.00s)
    get_gcs_test.go:129: err: dialing: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
--- FAIL: TestGCSGetter_ClientMode_notfound (0.00s)
    get_gcs_test.go:146: err: dialing: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
--- FAIL: TestS3Getter (1.73s)
    get_s3_test.go:36: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: 11DDA457B56600C3, host id: sA7AQAoVBnEgNG2bf1zpLA/58Mgsv37rbhXssJp60ENTlgvrVY5mV4l++/B8dUoShECJwAyOQC0=
--- FAIL: TestS3Getter_subdir (0.31s)
    get_s3_test.go:54: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: 69B5231C0C23B5C6, host id: RO7mQylIrYGEc1ce8QUUQIMUQk+4rxTOpLPiAmPLYs/CzewexSuTqlI1r/iwnNPuGBn7XUc+icQ=
--- FAIL: TestS3Getter_GetFile (0.31s)
    get_s3_test.go:73: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: FBD43BFAA71F4590, host id: p7w2W/ncQ8JK9jVNh3Lg0JTixpGQsDXqzMhtxujON8OAqidkhax0XCfDNe0uwzC8APT5EZ5tfUU=
--- FAIL: TestS3Getter_ClientMode_dir (0.28s)
    get_s3_test.go:121: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: 080AC40FE6783DD3, host id: EhDdfO8u7kPVD0fCGpxyNC2dSDGeK3anAW84axd0lOLnZkD3IStbXr7yDK3Do+RgR4NAC9A+9Ro=
--- FAIL: TestS3Getter_ClientMode_file (0.32s)
    get_s3_test.go:135: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: 6F264D4D121ACDD0, host id: BPmZUCpIfLZ1KTBnsWOUv8jhEfleTYWIRVpu/+RncCXKaOUe0KWED2ff8WkyDWX93Xgsg4L/2gU=
--- FAIL: TestS3Getter_ClientMode_notfound (0.29s)
    get_s3_test.go:153: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: A00E63C4C3393C66, host id: BOIwyoe400jgx9TikssmBTJA9b7eRK/DYyI4RIGfQS91klBRXGrxNnsOUGJrfztYr2rdmdZgQkk=
--- FAIL: TestS3Getter_ClientMode_collision (0.34s)
    get_s3_test.go:168: err: InvalidAccessKeyId: The AWS Access Key Id you provided does not exist in our records.
        	status code: 403, request id: 252BD05F6E948683, host id: a+Ai//NKt8yP/+PI1ZzJu61xdE+7WS+ODM/oLtDIYoKa7VEqSFmzctFZDmIPjGt+VCooCMzxAF8=
FAIL
FAIL	github.com/hashicorp/go-getter	21.062s

[prologic]

Oh on 2nd thoughts the above failures are likely caused by my not having said Google Cloud and AWS credentials. However I'm not going to go setup accounts just to make those pass :) (they should be mocking out externals calls to those APIs)

[prologic]

🏓