WIP: add seekable tar pack/unpack

[ecordell]

(unpack doesn't currently actually unpack the files, it just writes out their mediatypes)

$ ./bin/opm alpha pack internal/declcfg/testdata/valid > test.stargz
$ ./bin/opm alpha unpack test.stargz
internal/declcfg/testdata/valid/etcd.yaml application/vnd.opm.declcfg.json
internal/declcfg/testdata/valid/cockroachdb.json application/vnd.opm.declcfg.json
internal/declcfg/testdata/valid/unrecognized-schema.json application/vnd.opm.declcfg.json

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ecordell

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[codecov[bot]]

Codecov Report

Merging #664 (673bef4) into master (68c1758) will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #664      +/-   ##
==========================================
- Coverage   48.64%   48.63%   -0.02%     
==========================================
  Files          88       88              
  Lines        7672     7672              
==========================================
- Hits         3732     3731       -1     
- Misses       3205     3206       +1     
  Partials      735      735              

Impacted Files	Coverage Δ
pkg/registry/query.go	61.07% <0.00%> (-0.68%)	:arrow_down:

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[openshift-ci[bot]]

@ecordell: PR needs rebase.

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