Consider API redirection for the default variant

Example: I hit /api/v1/plugins/extractors/tap-stackexchange (currently a 404) and I get redirected to /api/v1/plugins/extractors/tap-stackexchange--meltanolabs (the default variant).

This is required until we have fuzzy search/match (e.g. meltano#3514 (comment 953679207)), otherwise for the plugin to be found either

1. users have to always pass --variant to meltano add or
2. meltano add hits the /index, and looks up the API URL and for the default variant if no particular one is requested