refactor(composer): Use string array for registry urls (#4180)

lib/datasource/packagist/index.js

lib/manager/common.ts

lib/manager/composer/artifacts.ts

lib/manager/composer/extract.ts

lib/manager/pip_requirements/extract.ts

test/datasource/packagist.spec.js

test/manager/composer/__snapshots__/extract.spec.ts.snap

test/manager/composer/_fixtures/composer1.json

test/manager/composer/_fixtures/composer2.json

test/manager/composer/_fixtures/composer3.json

test/manager/composer/_fixtures/composer4.json

test/manager/composer/_fixtures/composer5.json

test/manager/composer/artifacts.spec.ts

test/manager/composer/extract.spec.ts

test/workers/repository/process/lookup/index.spec.js