Repository file URLs are not checked for existence

[martinmcclure]

If you try to load a package using a cypress URL that points to a non-existent directory, the package loader thinks it sees an empty directory. If you already have that package loaded, the result is that all code from that package is deleted.

Both repository and package URLs should be checked for existence in the file system before the loader agrees to take any action on the load.

(I'm on the portability branch).

[martinmcclure]

Also, it looks like trying to load from a non-existent repository directory creates that directory. This is probably desirable when writing (you might be starting a new repository on disk), but not when loading.

[martinmcclure]

The culprit for the latter is CypressFileSystemRepository>>initializeUrl:andAlias:, which is sending #ensureDirectoryPathExists. Since it then tries to read the properties file, perhaps it should instead be erroring at this point when the directory does not exist.