cargo private

Re-enable compatibility with readonly CARGO_HOME Previously Cargo would attempt to work as much as possible with a previously filled out CARGO_HOME, even if it was mounted as read-only. In #6880 this was regressed as a few global locks and files were always attempted to be opened in writable mode. This commit fixes these issues by correcting two locations: * First the global package cache lock has error handling to allow acquiring the lock in read-only mode inaddition to read/write mode. If the read/write mode failed due to an error that looks like a readonly filesystem then we assume everything in the package cache is readonly and we switch to just acquiring any lock, this time a shared readonly one. We in theory aren't actually doing any synchronization at that point since it's all readonly anyway. * Next when unpacking package we're careful to issue a `stat` call before opening a file in writable mode. This way our preexisting guard to return early if a package is unpacked will succeed before we open anything in writable mode. Closes #6928