Move AccountClient to libs/command (#2447)

## Changes
Move the AccountClient reader and setter from the root package to
`libs/command`.

## Why
This allows us to read the account client set in the context in all CLI
packages. This was not possible before because using
`root.AccountClient` would often result in an import cycle.

This is a natural follow-up to
#2440 and
#2444.

## Tests
Existing tests and one new unit test.

NO_CHANGELOG=true

Author: shreyas-goenka

.codegen/service.go.tmpl

@@ -241,7 +241,7 @@ func new{{.PascalName}}() *cobra.Command {
 	cmd.PreRunE = root.Must{{if .Service.IsAccounts}}Account{{else}}Workspace{{end}}Client
 	cmd.RunE = func(cmd *cobra.Command, args []string) (err error) {
 		ctx := cmd.Context()
-		{{if .Service.IsAccounts}}a := root.AccountClient(ctx){{else}}w := command.WorkspaceClient(ctx){{end}}
+		{{if .Service.IsAccounts}}a := command.AccountClient(ctx){{else}}w := command.WorkspaceClient(ctx){{end}}
 		{{- if .Request }}
 			{{ if $canUseJson }}
 			if cmd.Flags().Changed("json") {