File URL's are not handled correctly in Windows systems

[pablochacin]

The file cache creates a file URL to the object of the form: file:///path/to/object (see Store and Get methods)

In windows systems, the resulting URL is file://C:/path/to/object. When this URL is parsed in the Download method, it throws an error because the C: component is interpreted as a host (with a missing port).

One solution is to implement this proposal: net/url: add FromFilePath and ToFilePath as platform dependent functions (with different implementations in Linux and Window systems).

An implementation of this proposal can be found here: https://go-review.googlesource.com/c/vuln/+/438175/7/internal/web/url.go