The resolved absolute path, or undefined when the specifier isn't relative or doesn't resolve to a real file.
Deliberately narrow: only handles a direct relative specifier resolving to
a real .ts/.tsx file on disk. Bare/package specifiers, namespace
imports, and anything requiring real module resolution (re-export chains,
exports map lookups, etc.) return undefined -- the caller treats that
as "couldn't statically link" and warns rather than guesses, the same
philosophy evaluateLiteral already uses for non-literal expressions.
Resolves a relative import specifier (as written in source:
"./schema.js", matching this codebase's own convention of.js-suffixed relative imports pointing at.tssource files) to an absolute file path, relative to the file that contains the import.