Put local imports in a separate import group

[corneliusweig]

As pointed out by @ahmetb here #195 (comment), imports are not sorted correctly. This PR applies standard sort order so that local imports are separated in their own import group. In addition, it also configures the linters to fail if imports are not sorted as expected.

[codecov-io]

Codecov Report

Merging #222 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #222   +/-   ##
=======================================
  Coverage   54.35%   54.35%           
=======================================
  Files          13       13           
  Lines         701      701           
=======================================
  Hits          381      381           
  Misses        269      269           
  Partials       51       51

Impacted Files	Coverage Δ
pkg/installation/install.go	36.36% <ø> (ø)	⬆️
pkg/index/indexscanner/scanner.go	67.34% <ø> (ø)	⬆️
pkg/index/validate.go	94.44% <ø> (ø)	⬆️

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[ahmetb]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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