Path completions from subpath exports containing / have broken replacement range

[andrewbranch]

Fourslash test:

/// <reference path="fourslash.ts" />

// @module: preserve
// @moduleResolution: bundler
// @allowJs: true

// @Filename: /node_modules/repo/package.json
//// {
////   "name": "repo",
////   "exports": {
////     "./foo/bar": "./src/bar.js"
////   }
//// }

// @Filename: /node_modules/repo/src/bar.js
//// export {};

// @Filename: /main.js
//// import { } from "repo/[|f/**/|]";

verify.completions({
  marker: "",
  isNewIdentifierLocation: true,
  exact: [
    { name: "foo/bar", kind: "script", kindModifiers: "", replacementSpan: test.ranges()[0] },
  ],
});

The actual replacement range is the full module specifier. (VS Code shows nothing since the text of the replacement range doesn’t match the insertion text.) The root cause is #41412. At the time, only full module specifiers from compilerOptions.paths were able to return slashes (I guess?).

The replacement range logic is really overcomplicated and based on weird heuristics like this. I think it would be simpler and more reliable for path completions to always return entries with replacement ranges equal to the entire string literal (and with text starting from the beginning of the string literal), instead of sometimes returning fragments intended to be inserted after the last slash. However, the existing code makes this really difficult, as nearly all functions directly mutate the set of results without having any knowledge of what’s already been typed in the string literal.

Originally reported at #56412 (comment)