Module zips are stored under the module's PURL, which lowercases golang
names, and under the filename {last element}@{version}.zip. So
github.com/Sirupsen/logrus and github.com/sirupsen/logrus shared one
cache entry although upstream serves different zips, and the second one
requested failed go's checksum check.
A module path with capitals now gets a hash of its exact spelling in the
filename. All-lowercase paths keep the plain name, so existing cache
entries stay valid.
A capitalised module cached before this change sits under its lowercase
twin's plain filename. When a cached zip's upstream URL names a different
module path, it is discarded and fetched again. Only the module path is
compared, so a cache filled from another upstream.go host is kept.
Fixes#466
