NPM-style paths break remote repository proxying

[jdcasey]

This has popped up when using httprox to proxy a NPM client. NPM requests the package descriptor eg:

/gulp-size

and then retrieves associated files such as:

/gulp-size/-/gulp-size-1.3.0.tgz

The problem is that, with traditional filesystem storage, the first request prevents the creation of the directory structure for storing the cached content of the second request.

For httprox proxies, we should be able to resort to hashed path generation. In preparation for supporting other types of content natively (i.e. not via httprox) it would be good to build in this per-repo path-generation override to the Indy core.