debuginfod: DEBUGINFOD_URLS should accept scheme-free urls

debuginfod: DEBUGINFOD_URLS should accept scheme-free urls

Check scheme instead of effective url so that user may abbreviate
DEBUGINFOD_URL. Add one test for scheme free http url.

Notice that libcurl does not provide an almighty scheme free url
support, /path/to/something without FILE:// can not be recognized
in most circumstances, therefore for the neatness of our code
structure, DEBUGINFOD_ URL of scheme "FILE" must be input as URI.

Signed-off-by: Alice Zhang <alizhang@redhat.com>