Packer can only install plugins from github.com, with
releases.hashicorp.com consulted first for HashiCorp-published plugins.
This has been a long-standing gap for air-gapped and policy-restricted
environments (#11164): the source address parser already accepts any
hostname, but both existing getters reject non-github.com sources at
install time.
Add a remote plugin getter that installs plugins from the host named in
a required_plugins source address. The host serves the directory
structure of releases.hashicorp.com under the source's path: an
index.json listing versions, and per version a SHA256SUMS file, the
zips it lists, and - when the zip names carry no plugin protocol
version - the version's manifest.json. A plugin published on
releases.hashicorp.com is therefore mirrored as a verbatim copy of its
tree, with every checksum file and signature upstream-authored. A
plugin published as GitHub release assets is mirrored by copying each
release's assets into a version directory, renaming their SHA256SUMS
file to the unprefixed convention with content unchanged, and writing
an index.json listing the versions. Both kinds of content can be
served side by side by one host.
Getter selection happens per source address: github.com sources keep
the release and github getters unchanged, while any other host is
served by the remote getter over HTTPS. Sources with three or more
components are supported, up to the existing 16-component limit, so
nested artifact-repository paths and hosts that embed the upstream
origin in their path all resolve.
Version discovery, constraint solving, checksum verification, and the
binary naming rules match the existing getters. The installed filename
is rebuilt from validated checksum-file fields and never taken from the
server's response, checksum entries matching neither known naming shape
are rejected rather than guessed at, and nothing the remote metadata
supplies is used to fetch from another origin or path. index.json
parsing is covered by fixtures captured from the live releases API, so
a format change there fails tests rather than user installs.
Closes#11164
