refactor(function): findPR function(#4996) (#5129)

lib/platform/azure/index.ts

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket/index.ts

lib/platform/common.ts

lib/platform/github/index.ts

lib/platform/gitlab/index.ts

lib/workers/branch/check-existing.ts

lib/workers/repository/finalise/prune.ts

lib/workers/repository/master-issue.ts

lib/workers/repository/onboarding/branch/check.ts

test/platform/azure/index.spec.ts

test/platform/bitbucket-server/index.spec.ts

test/platform/bitbucket/index.spec.ts

test/platform/github/index.spec.ts

test/platform/gitlab/index.spec.ts

test/workers/repository/master-issue.spec.ts