Importers: Consolidate list of supported/available importers

[daledupreez]

Details

We have multiple places in Calypso that track permitted/enabled importers, including the following:

• getConfig() and getImporters() in client/lib/importer/importer-config.tsx
• The logic in getFinalImporterUrl() in client/landing/stepper/declarative-flow/internals/steps-repository/import/helper.ts includes a local productImporters constant
• The ListStep component in client/blocks/import/list/index.tsx includes a hard-coded list of primary and secondary import platforms

Checklist

• Confirm which platforms should be enabled during import flows
• Confirm which location we're going to use as our source of truth
• Update callers to use the primary location

Related

No response