bug#49982: 27.2; ispell.el fails to find a Hunspell dictionary to use as default despite ispell-dictionary being set

[Kisaragi Hiu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

This configuration should be everything that's needed for ispell.el to
work with Hunspell, regardless of system locale:

     (setq ispell-program-name (executable-find "hunspell")
           ispell-dictionary "en_US"))

However, when system locale (the LANG environment variable) does not 
have a corresponding Hunspell dictionary, 
`ispell-find-hunspell-dictionaries` returns the error "Can't find 
Hunspell dictionary with a .aff affix file", despite ispell-dictionary 
being set.

ispell.el relies on Hunspell to load a default and report it, but
Hunspell just errors out if it can't find a dictionary for the system
locale. And because ispell.el is trying to get Hunspell's default
dictionary, it doesn't pass `ispell-dictionary' onto Hunspell.

This behavior is surprising. If `ispell-dictionary` is non-nil, that
means the user has already specified their preferred dictionary, and it
should not matter that Hunspell cannot find the dictionary it would use
when a preferred dictionary isn't specified.

It's ispell.el that needs to be fixed here because the user specifies
their preference in Emacs, and it is its job to communicate that
preference to Hunspell.

`ispell-find-hunspell-dictionaries` should pass "-d
${ispell-dictionary}" to Hunspell if `ispell-dictionary` is set. This 
invocation:

     hunspell -d "en_US" -D /dev/null

works as expected regardless of the system locale.

* System info

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27,
  cairo version 1.17.4) of 2021-03-27 built on juergen Windowing system
  distributor 'The X.Org Foundation', version 11.0.12013000 System
  Description: Arch Linux

Hunspell 1.7.0; hunspell -D is

     SEARCH PATH:
 
.::/usr/share/hunspell:/usr/share/myspell:/usr/share/myspell/dicts:/Library/Spelling:/home/kisaragi-hiu/.openoffice.org/3/user/wordbook:/home/kisaragi-hiu/.openoffice.org2/user/wordbook:/home/kisaragi-hiu/.openoffice.org2.0/user/wordbook:/home/kisaragi-hiu/Library/Spelling:/opt/openoffice.org/basis3.0/share/dict/ooo:/usr/lib/openoffice.org/basis3.0/share/dict/ooo:/opt/openoffice.org2.4/share/dict/ooo:/usr/lib/openoffice.org2.4/share/dict/ooo:/opt/openoffice.org2.3/share/dict/ooo:/usr/lib/openoffice.org2.3/share/dict/ooo:/opt/openoffice.org2.2/share/dict/ooo:/usr/lib/openoffice.org2.2/share/dict/ooo:/opt/openoffice.org2.1/share/dict/ooo:/usr/lib/openoffice.org2.1/share/dict/ooo:/opt/openoffice.org2.0/share/dict/ooo:/usr/lib/openoffice.org2.0/share/dict/ooo
     AVAILABLE DICTIONARIES (path is not mandatory for -d option):
     ... [truncated]
     /usr/share/hunspell/en_US-large
     ... [truncated]

* Reproduction

- Notice how Hunspell does not return LOADED DICTIONARY under, for 
example, ja_JP:

     export LANG=ja_JP
     hunspell -D /dev/null
     # Output:
     # ... [truncated]
     # Can't open affix or dictionary files for dictionary named "ja_JP".

- Now, in Emacs with LANG set to ja_JP, set ispell up with Hunspell as 
usual.

     (setq ispell-program-name (executable-find "hunspell")
           ispell-dictionary "en_US"))

- Observe the error.

     (ispell-start-process)
     ;; -> ispell-find-hunspell-dictionaries: Can^[$B!G^[(Bt find Hunspell 
dictionary with a .aff affix file