fix(lagoon): keep Status plugin-only; migrate:status adds the framework sets itself (WR-12)

57a849d made lagoon.Status read migrationSets, which changed its result
for consumers: the framework sets came first and a plugin with no
migrations lost its row. Status is back to its pre-57a849d behaviour.
migrate:status (table and --json) now reads the unexported setStatus,
which lists the framework sets and the plugins from migrationSets, so it
still agrees with migrate --pretend. Both share historyRows.

- TestStatusPluginRows pins Status: plugin rows only, an empty row for a
  plugin with no migrations or no history table, none without HasMigrations
- TestMigrateStatusJSON also checks the table lists the framework sets
- TestTwoPluginMigrationSetsIsolated and TestDocsMigrate are back to the
  plugin-only rows

modules/lagoon/README.md

@@ -135,7 +135,7 @@ func (p *Plugin) Migrations() []*gormigrate.Migration {
 | `lagoon.DSN` | Reads `database.dsn` from config. |
 | `lagoon.Migrate` | Runs framework and plugin migrations in order. |
 | `lagoon.RollbackLast` | Rolls back the last migration of one plugin. |
-| `lagoon.Status` | Lists the applied migration IDs of every set `lagoon.Migrate` applies, framework sets first (under their history IDs, such as `summercms.attach`), then each plugin with migrations, as `lagoon.StatusRow` values. |
+| `lagoon.Status` | Lists applied migration IDs per plugin as `lagoon.StatusRow` values: one row for each plugin that implements `pact.HasMigrations`, in plugin order; a plugin with no migrations, or without a history table yet, gets a row without IDs. The framework's own sets are not listed; `migrate:status` adds them. |
 | `lagoon.DatabaseLabel` | Describes a DSN for a person as `host:port/name` (parsed with `pgconn.ParseConfig`), never the user or password; `not configured` when empty, `configured` when it does not parse. Used by the startup summaries of `serve` and `queue:work`. |
 | `lagoon.PendingMigrations` | Lists the migrations `lagoon.Migrate` would apply, as `lagoon.MigrationRef` values (`Plugin`, `ID`): the framework sets first, then each plugin in order, each in declaration order. A missing history table means the whole set is pending. Read-only; `serve` reports the count in its startup summary. |
 | `lagoon.HistoryTableName` | Returns the gormigrate history table for a plugin ID. |