refactor: consolidate platform ts (#4283)

lib/platform/azure/index.ts

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket/index.ts

lib/platform/bitbucket/utils.ts

lib/platform/common.ts

lib/platform/github/index.ts

lib/platform/gitlab/index.ts

lib/platform/index.ts

test/platform/azure/index.spec.ts

...latform/bitbucket-server/__snapshots__/index.spec.ts.snap

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

test/platform/bitbucket/__snapshots__/index.spec.ts.snap

test/platform/bitbucket/index.spec.ts

test/platform/github/__snapshots__/index.spec.ts.snap