autoconf: Pretend `CONFIG_MODULES` is always on

- Unconditionally inject `CONFIG_MODULES` make variable
  and `#define CONFIG_MODULES` to Kbuild in `ZFS_LINUX_COMPILE`
  autoconf function to emulate loadable kernel modules support.
  This allows OpenZFS to perform Linux checks despite
  `CONFIG_MODULES=n` in the actual Linux config.

- Add `ZFS_AC_KERNEL_CONFIG_MODULES` check which encompasses
  the logic from `ZFS_AC_KERNEL_TEST_MODULE` with additional
  diagnostic messages to the user

- Removed `ZFS_AC_KERNEL_TEST_MODULE` as it merely duplicates
  every check in `ZFS_AC_KERNEL_CONFIG_DEFINED`

- Moved `ZFS_AC_MODULE_SYMVERS` after `ZFS_AC_KERNEL_CONFIG_DEFINED`
  so the user has a chance to see the proper diagnostic from the
  steps before.

A workaround for Linux's

```
commit 3e3005df73b535cb849cf4ec8075d6aa3c460f68
Author: Masahiro Yamada <[email protected]>
Date:   Wed Mar 31 22:38:03 2021 +0900

kbuild: unify modules(_install) for in-tree and external modules

If you attempt to build or install modules ('make modules(_install)'
with CONFIG_MODULES disabled, you will get a clear error message, but
nothing for external module builds.

Factor out the modules and modules_install rules into the common part,
so you will get the same error message when you try to build external
modules with CONFIG_MODULES=n.

Signed-off-by: Masahiro Yamada <[email protected]>
```

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: szubersk <[email protected]>
Closes #10832
Closes #13361 

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