Pass sourcedir to rpmspec when specfile is parsed
    
    In many cases, specfile parsing in class SpecFile failed when
    the specfile contained local sources like 'Source2: local.txt'.
    This happened because rpm sourcedir configuration was not passed
    to utility rpmspec used for parsing, leading it to default
    to '~/rpmbuild/SOURCES', which is not correct for many layouts
    supported by rpkg.
    Fixed by passing sourcedir correctly.
    
    Fixes: #559
    Relates: https://pagure.io/rpkg/pull-request/564
    Merges: https://pagure.io/rpkg/pull-request/574
    
    Signed-off-by: Otto Urpelainen <oturpe@iki.fi>