Cased modules not served when casing doesn't match

[shoenig]

For example,
if github.com/pagerduty/[email protected]
is added to the proxy, requests for the "true" module name of:
github.com/PagerDuty/[email protected]
will result in 404s. However, it's then also not possible to add the true-er module name, because as far as modprox is concerned it already exists (probably doing an ignore-case string comparison).