Move implementation details to internal

[ferhatelmas]

move all packages into internal except pkg/index and pkg/constants.

fixes #291

[ahmetb]

HUGE if we can get it in ASAP before it starts introducing merge conflicts everywhere :)

[codecov-io]

Codecov Report

Merging #422 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #422   +/-   ##
=======================================
  Coverage   56.41%   56.41%           
=======================================
  Files          19       19           
  Lines         927      927           
=======================================
  Hits          523      523           
  Misses        349      349           
  Partials       55       55

Impacted Files	Coverage Δ
internal/download/verifier.go	100% <ø> (ø)
internal/installation/install.go	42.42% <ø> (ø)
internal/installation/upgrade.go	0% <ø> (ø)
internal/receiptsmigration/migration.go	46.15% <ø> (ø)
cmd/validate-krew-manifest/main.go	54.54% <ø> (ø)	⬆️
internal/pathutil/pathutil.go	100% <ø> (ø)
internal/index/validation/validate.go	92.1% <ø> (ø)
internal/installation/move.go	33.09% <ø> (ø)
internal/installation/platform.go	79.16% <ø> (ø)
internal/version/version.go	100% <ø> (ø)
... and 9 more

---

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[ahmetb]

I'll go ahead with this, seeing no issues locally.

I'll send a PR to move cmd/* commands to v0.3.3 later.

[ahmetb]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, ferhatelmas

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