stdlib: resolve absolute script paths independently of sys.path

[ac1982]

RunFile fails to open an existing absolute script path with the default compile options: resolveRunPath joins it to "." first, which removes the leading slash before the absolute-path check. With an empty sys.path, it never attempts the file at all.

Resolve absolute inputs independently of search directories by joining them only to an empty prefix. Relative-path search and existing error handling are unchanged. Add an execution regression using a real temporary script with both default options and an empty sys.path.

Fixes #259. This is independent of the nil-module-pointer fix in #260 and is based directly on main.

Validation on Linux/arm64:

• Both absolute-path cases fail with FileNotFoundError before the fix and pass afterward.
• Go 1.27.1 full project runner with -tags=ci -race -coverpkg=github.com/go-python/gpython/... passes.
• Go 1.18.10 targeted test and Staticcheck 2022.1.3 across all packages pass.
• goimports and git diff --check pass. Windows/macOS runtime behavior was not tested.

Author registration: go-python/license#24.

AI assistance: this fix and its regression tests were developed and verified with an AI coding agent.