getMappedPath only resolves to files that exist. This does not
work when a TSConfig path alias resolves to a module folder
containing built assets, and described by a package.json, e.g.
foo/dist.

If the tsconfig path is mapped to a folder, then getMappedPath
returns undefined as the folder is not a file. However, in somes
cases it is ok for this to resolve as the package.json contains
a `typings` entry that references a .d.ts file, which is
correctly handled by enhanced-resolve

Use `path.sep` as the file separator instead of `/` so that this
works on both Windows and Unix.

Rename input parameter from `path` to `modulePath` so that it
doesn't override `node:path`

Co-authored-by: JounQin <admin@1stg.me>