insights: add new discovery package for locating insights (and use it)

[emidoots]

This will be used both from the GraphQL resolver layer, as well as the
background workers - both of which need to scan the user/org/global settings
for insights defined within.

Also @efritz this cleans up the settings store mocks :)

[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff a2311ca...bd4f455.

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@felixfbecker

enterprise/internal/insights/discovery/discovery.go enterprise/internal/insights/discovery/discovery_test.go enterprise/internal/insights/discovery/gen.go enterprise/internal/insights/discovery/mock_setting_store.go enterprise/internal/insights/resolvers/insight_connection_resolver.go enterprise/internal/insights/resolvers/insight_connection_resolver_test.go enterprise/internal/insights/resolvers/insight_series_resolver_test.go