Check if paths already exist when views or layouts are being installed.

This came up in #1238...

@behindthepage in https://github.com/backdrop/backdrop-issues/issues/1238#issuecomment-142...

... I think paths should be checked when views or layouts are being installed. I think it is more likely to duplicate a path when installing a view or a layout than having conflicting config file names and this too can be solved with a bit more code during the copying of the config files. Maybe we can change the new path to duplicate_0 and set a message during install.

@quicksketch in https://github.com/backdrop/backdrop-issues/issues/1238#issuecomment-142...

Let's make an issue for this one in particular. I think we may be able to do a check, but doing a validation before installation may be difficult because each type of thing makes a menu path differently. It's only when they're all consolidated into hook_menu() that you have the full list. And then each module could have duplicates within it as well (two views at the same path, two layouts at the same path, etc.)