bug#42543: lint failure for packages on load path specified with -L

[Jack Hill]

Hi Guix,

I'm running guix from commit 30aa5dd7e7180d163d409b080bf89e8a15a5ba4d. 
I've created a package in local directory lint-test. `guix lint` errors 
when processing this package, but other guix commands like build and show 
work as expected. A session with these commands:

```
jackhill@alperton ~$ guix lint -L lint-test my-hello
Backtrace:y-hello@2.10 [formatting]...
           8 (primitive-load "/home/jackhill/.config/guix/current/bi…")
In guix/ui.scm:
  1974:12  7 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1736:10  6 (with-exception-handler _ _ #:unwind? _ # _)
In guix/store.scm:
   631:22  5 (thunk)
In srfi/srfi-1.scm:
    634:9  4 (for-each #<procedure 7f40c0484020 at guix/scripts/lin…> …)
In guix/scripts/lint.scm:
     60:4  3 (run-checkers _ _ #:store _)
In srfi/srfi-1.scm:
    634:9  2 (for-each #<procedure 7f40b2ab53f0 at guix/scripts/lin…> …)
    241:2  1 (for-each _ _)
In ice-9/boot-9.scm:
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
In procedure for-each: Wrong type argument: #f
jackhill@alperton ~$ guix build -L lint-test my-hello
/gnu/store/h21bncim3hshjpfbs1dn4kvg1ri6zb7z-my-hello-2.10
jackhill@alperton ~$ guix show -L lint-test my-hello
name: my-hello
version: 2.10
outputs: out
systems: x86_64-linux i686-linux
dependencies: 
location: lint-test/my-hello.scm:8:2
homepage: https://www.gnu.org/software/hello/
license: GPL 3+
synopsis: Hello, GNU world: An example GNU package 
description: GNU Hello prints the message "Hello, world!" and then
+ exits.  It serves as an example of standard GNU coding practices.  As
+ such, it supports command-line arguments, multiple languages, and so on.
```

my-hello.scm looks like:

```
(define-module (my-hello)
  #:use-module (guix build-system gnu)
  #:use-module (guix download)
  #:use-module (guix licenses)
  #:use-module (guix packages))

(define-public my-hello
  (package
    (name "my-hello")
    (version "2.10")
    (source (origin
              (method url-fetch)
              (uri (string-append "mirror://gnu/hello/hello-" version
                                  ".tar.gz"))
              (sha256
               (base32
                "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i"))))
    (build-system gnu-build-system)
    (synopsis "Hello, GNU world: An example GNU package")
    (description
     "GNU Hello prints the message \"Hello, world!\" and then exits.  It
serves as an example of standard GNU coding practices.  As such, it supports
command-line arguments, multiple languages, and so on.")
    (home-page "https://www.gnu.org/software/hello/";)
    (license gpl3+)))
```

Best,
Jack