This change adds a simple hostname validation check to validate that a
plugins source address is github.com before continuing with the Get
call. An issue was encountered when using a hostname different from
github.com, where the getter would continue to pull a plugin from GitHub
even if the hostname was something like "example.com". See log details
below.
Before change
```
2021/02/16 12:49:41 [TRACE] fetching checksums file for the "0.0.2" version of the example.com/hashicorp/docker plugin in
"/home/wilken/.packer.d/plugins/example.com/hashicorp/docker"...
2021/02/16 12:49:41 [DEBUG] github-getter: getting "https://github.com/hashicorp/packer-plugin-docker/releases/download/v0.0.2/packer-plugin-docker_v0.0.2_SHA256SUMS"
2021/02/16 12:49:42 [TRACE] Ignoring remote binary packer-plugin-docker_v0.0.2_x5.0_linux_arm64.zip, wrong system, expected
2021/02/16 12:49:42 [TRACE] About to get: packer-plugin-docker_v0.0.2_x5.0_linux_amd64.zip
2021/02/16 12:49:42 [DEBUG] github-getter: getting "https://github.com/hashicorp/packer-plugin-docker/releases/download/v0.0.2/packer-plugin-docker_v0.0.2_x5.0_linux_amd64.zip"
```
After change
```
2021/02/16 13:36:32 [TRACE] for plugin example.com/hashicorp/docker found 0 matching installation(s)
2021/02/16 13:36:32 [TRACE] getting available versions for the the example.com/hashicorp/docker plugin
2021/02/16 13:36:32 [TRACE] &{%!q(*github.Client=<nil>) "packer-getter-github-1.7.0-dev"} getter could not get release: example.com/hashicorp/docker doesn't appear to be a valid github.com source address; check source and try again.
2021/02/16 13:36:32 [DEBUG] will try to install: []
2021/02/16 13:36:32 [INFO] (telemetry) Finalizing. no release version found for the example.com/hashicorp/docker plugin matching the constraint(s): ">=v0.0.2"
2021/02/16 13:36:32 waiting for all plugin processes to complete...
```
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