lib/options/showOption: fix quoting of attr-names that are not identifiers

Personally, I think that warnings such as

    warning: The option `services.redis.enable' defined in `/home/ma27/Projects/nixpkgs/test.nix@node-vm' has been renamed to `services.redis.servers..enable'.

are fairly confusing because of the `..` and it's more correct to
actually quote that. With this change the warning now looks like this:

    warning: The option `services.redis.enable' defined in `/home/ma27/Projects/nixpkgs/test.nix@node-vm' has been renamed to `services.redis.servers."".enable'.

While implementing that I realized that you'd have
a similar problem whenever you use attribute-names that aren't
identifiers, e.g.

    services.nginx.virtualHosts."example.org".locations."/".invalid = 23;

now results in the following error:

    error: The option `interactive.nodes.vm.services.nginx.virtualHosts."example.org".locations."/".invalid' does not exist. Definition values:
           - In `/home/ma27/Projects/nixpkgs/test.nix@node-vm': 23

Of course there are some corner-cases where this won't work: when
generating the manual, you display submodules like this:

    services.nginx.virtualHosts.<name>

Since `<name>` isn't a value, but an indicator for a submodule, it must
not be quoted. This also applies to the following identifiers:

* `*` for `listOf submodule`
* `<function body>` for `functionTo`

This might not be correct if you actually have a submodule with an
attribute name called `<name>`, but I think it's an improvement over the
current situation and for this you'd probably need to make even more
complex changes to the module system.

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