Add goimport local-prefixes configuration to .golangci.yml

[adityacs]

What this PR does / why we need it:

1. Add goimport local-prefixes configuration to .golangci.yml
2. Formatted imports unders pkg/ and cmd/
3. Updated CONTRIBUTING.md for Coding Standards

Which issue(s) this PR fixes:
Fixes #1348

Checklist

• Documentation added

[codecov-io]

Codecov Report

Merging #1710 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1710   +/-   ##
=======================================
  Coverage   64.07%   64.07%           
=======================================
  Files         121      121           
  Lines        9027     9027           
=======================================
  Hits         5784     5784           
  Misses       2841     2841           
  Partials      402      402

Impacted Files	Coverage Δ
pkg/logentry/stages/timestamp.go	87.34% <ø> (ø)	⬆️
pkg/logcli/output/default.go	100% <ø> (ø)	⬆️
pkg/logql/marshal/marshal.go	75.86% <ø> (ø)	⬆️
pkg/logql/range_vector.go	97.56% <ø> (ø)	⬆️
pkg/promtail/client/batch.go	94.73% <ø> (ø)	⬆️
cmd/fluent-bit/out_loki.go	5.79% <ø> (ø)	⬆️
pkg/distributor/ingestion_rate_strategy.go	88.88% <ø> (ø)	⬆️
pkg/querier/queryrange/stats.go	100% <ø> (ø)	⬆️
pkg/ingester/transfer.go	66.42% <ø> (+1.42%)	⬆️
pkg/logentry/stages/tenant.go	95.12% <ø> (ø)	⬆️
... and 34 more

[adityacs]

Build 2027 shows error when linter fails

[cyriltovena]

LGTM