completeFlakeRefWithFragment: handle names with dots

[ylh]

Motivation

remedies a FIXME:

nix build /etc/nixos#nixosConfigurations.w
# may now tab-complete to e.g.:
nix build /etc/nixos#nixosConfigurations.\"www.example.com\"

& subsequent completions into deeper attrpaths will preserve escaped quotes.

Context

{un,}escaping done in this PR errs on the side of simplicity to the point of being naïve (pretty easy to demonstrate that they are not strict inverses of each other), expectation being that tab completion is used interactively.

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