Fix rewrites parts of the plugin codebase to address known issues or
common workarounds used within plugins consuming the Packer plugin SDK.

```
~>  ./packer-sdc fix -check ../../../packer-plugin-tencentcloud
Found the working directory
/Users/wilken/Development/linux-dev/packer-plugin-tencentcloud
gocty Unfixed!

```

This change is major refactor of the fix command, and it's underlying
fixer interface. This change adds support for the `-diff` flag which
displays the diff between the unfixed and fixed files, if available.

Along with the refactor the ability to apply multiple fixes to the same
file without potential write conflicts where previous changes were
removed after reprocessing.

* Add a scan function that returns a list of files to apply a fix on
to provide flexibility in the future for collecting a list of files.

* Replace cmp.Diff with github.com/pkg/diff for better file diffs
* Return error when missing directory argument
* Add error handling when trying to read any scanned files