Small refactorings for better code structure

[corneliusweig]

This PR includes some small refactorings which originally were part of #195 . In particular:

• move the expected ending for manifests into a code constant
• check that this code constant is not used outside of constants.go
• move the logic to match a target platform out of installation and into the index package.

[codecov-io]

Codecov Report

Merging #223 into master will decrease coverage by 0.57%.
The diff coverage is 79.31%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
- Coverage   54.35%   53.78%   -0.58%     
==========================================
  Files          13       14       +1     
  Lines         701      701              
==========================================
- Hits          381      377       -4     
- Misses        269      273       +4     
  Partials       51       51

Impacted Files	Coverage Δ
pkg/installation/util.go	44.89% <100%> (-15.92%)	⬇️
cmd/validate-krew-manifest/main.go	58.33% <100%> (ø)	⬆️
pkg/index/indexscanner/scanner.go	67.34% <100%> (ø)	⬆️
pkg/index/platform.go	76% <76%> (ø)

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[ahmetb]

seems like this import should be on the next block. (conflicts with #222)

[ahmetb]

Small comments.
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ahmetb]

/lgtm