bug#59423: Invalid 'location' field generated in dovecot configuration

[mirai]

I'm also experiencing the same issue (guix describe: 
7e0ad0dd0f2829d6f3776648ba7c88acf9888d7a).

My guess is that 44554e7133aa60e1b453436be1e80394189cabd9 (which supersedes 
543d971ed2a1d9eb934af1f51930741d7cc4e7ef)
introduces a '%location' field which conflicts with 'dovecot-configuration' 
itself also having a field called 'location'.

In fact, interesting things happen if you define a configuration with a 
'location' field.
With 'guix repl':
```
$ guix repl
GNU Guile 3.0.8
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guix-user)> (use-modules (gnu services configuration))
(define-configuration FOO-configuration
  (name
   (string "aaa")
   "")
  (location
   (string "bbb")
   ""))
;;; <stdin>:2:0: warning: shadows previous definition of 
`%FOO-configuration-location-procedure' at <stdin>:2:0
;;; <unknown-location>: warning: shadows previous definition of 
`FOO-configuration-location' at <unknown-location>
;;; <stdin>:2:0: warning: possibly unbound variable `serialize-string'
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
error: serialize-string: unbound variable

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guix-user) [1]>
```

Code snippet for convenience:
```
(use-modules (gnu services configuration))
(define-configuration FOO-configuration
  (name
   (string "aaa")
   "")
  (location
   (string "bbb")
   ""))
```