new lints: path append is not path resolution

[roberth]

Is your feature request related to a problem?

path + path has jank.

Misunderstanding of path literals:

nix-repl> ./. + ../foo
/home/user/src/nix/home/user/src/foo

Simply objectionable:

nix-repl> ./. + ./foo  
/home/user/src/nix/home/user/src/nix/foo

Dubious pattern, but pretty sure this is in use, so this should be a separate flag:

nix-repl> ./. + /foo  
/home/user/src/nix/foo

Missing slash in string breaks path-level abstraction boundary:

nix-repl> ./. + "../foo"
/home/user/src/nix../foo

Appended "/.." is resolved without following symlink.
This is probably ok most of the time, so consider

• separate lint flag
• check if following symlinks would have been different

Should recommend dirOf to make it actionable.

nix-repl> ./. + "/../foo"
/home/user/src/foo

Proposed solution

A set of new lint- options that check for the above.

Alternative solutions

Additional context

• Mostly unrelated to doc(lint-absolute-path-literals): edit #15887

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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