Refactor existing index path functions to support multiple indexes later on

[chriskim06]

Add a name parameter to the index path functions (and behind the X_KREW_ENABLE_MULTI_INDEX variable) to return the path for the specified index, with "default" being implicit. Refactor the existing calls to use an empty string for now that gets ignored when the variable is unset.

The integration tests fail when X_KREW_ENABLE_MULTI_INDEX is set since the other functionality doesn't exist yet.

Related issue: #483

[codecov-io]

Codecov Report

Merging #510 into master will increase coverage by 0.24%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
+ Coverage   58.87%   59.12%   +0.24%     
==========================================
  Files          23       23              
  Lines         997     1003       +6     
==========================================
+ Hits          587      593       +6     
  Misses        350      350              
  Partials       60       60

Impacted Files	Coverage Δ
internal/environment/environment.go	89.74% <100%> (+1.86%)	⬆️
internal/indexmigration/migration.go	65.21% <100%> (ø)	⬆️
internal/info/info.go	100% <100%> (ø)	⬆️
internal/receiptsmigration/migration.go	46.15% <50%> (ø)	⬆️

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[corneliusweig]

Overall looks good to me. However, you need to test both behaviors at the same time. I.e. you need to set up the X_KREW_ENABLE_MULTI_INDEX env var in tests, so that you can test both branches.

[ahmetb]

Refactor the existing calls to use an empty string for now that gets ignored when the variable is unset.

In general, if you use "default" string everywhere, you can finish refactoring those pieces later. Whereas "" is harder to find and replace.

[ahmetb]

LGTM but one major point surfaced up in all the patch.

[corneliusweig]

Looks good overall. I have only some smaller nits, then let's merge this.

[corneliusweig]

Ok, let's get this in. Once the conflict is solved, I'll merge.

[ahmetb]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, chriskim06

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