`file://` URI resolution doesn't work in pkger when running on Windows

Windows paths like `C:\foo\bar` must be translated into a form like `file:///C/foo/bar` to be valid URIs. The pkger currently doesn't account for this, so when running on Windows it directly tries (and fails) to open `/C/foo/bar` when given a `file://` URI.

Start by locating pkger's handling of file:// URIs and reproduce the Windows path case described in the issue. The work is done when a path such as C:\foo\bar is translated to file:///C/foo/bar and can be opened successfully on Windows.

Written by the indexing model from the issue text.