* removed packer.Cache and references since packer.Cache is never used except in the download step. The download step now uses the new func packer.CachePath(targetPath) for this, the behavior is the same.
* removed download code from packer that was reimplemented into the go-getter library: progress bar, http download restart, checksuming from file, skip already downloaded files, symlinking, make a download cancellable by context.
* on windows if packer is running without symlinking rights and we are getting a local file, the file will be copied instead to avoid errors.
* added unit tests for step_download that are now CI tested on windows, mac & linux.
* files are now downloaded under cache dir `sha1(filename + "?checksum=" + checksum) + file_extension`
* since the output dir is based on the source url and the checksum, when the checksum fails, the file is auto deleted.
* a download file is protected and locked by a file lock,
* updated docs
* updated go modules and vendors
Tested and verified using "make website".
Fix up the vmware, vbox and qemu builder and communicator docs.
Sort more things in alphabetical order.
Use backtick delimiters for values.
Wrap long lines to 80 characters.
Show actual default value for boot_wait.
Copy vrdp_bind_address info from vbox iso to vbox ovf builder.
Show defaults for vmx_remove_ethernet_interfaces.
Show default port values for ssh in vbox ovf builder.
Remove executable bit on openstack builder doc.
Show default for skip_compaction in vmware vmx builder.
Add a bunch of missing periods.
Remove a few redundant commas.
