Split fetch from jobsearch into a separate fetcher command, and renamed fetch once to fts-once

job-search no longer has a fetch command. Its subcommands are now in
cmd/fetcher, which reads the same configuration: "fetcher service" runs
every enabled source's fetcher on its interval until stopped, and
"fetcher fts-once" (was "fetch once") fetches from Find a Tender once and
prints the releases as JSON. The new namea say what the commands do,
because service runs every source while fts-once fetches only from Find a
Tender.

The code stays in internal/cli/fetch, where NewCommands replaces the fetch
parent command. once.go is now fts_once.go and commandOnce is
commandFTSOnce. The README's usage, the config README and the find_a_tender
api_root comment now give the new command names.

internal/config/conf_source_findatender.go

@@ -7,7 +7,7 @@ const JSCKSourceFindATender string = "find_a_tender"
 
 // JSCSourceFindATender configures fetching from the Find a Tender
 // OCDS API at APIRoot. Empty: the source fails to start if it is enabled,
-// and `fetch once` fails.
+// and `fetcher fts-once` fails.
 type JSCSourceFindATender struct {
 	JSCSourcesSource
 	APIRoot string `cfg:"api_root" default:"https://www.find-tender.service.gov.uk/api/1.0"`