Allow to not download local files and simply reference them 'inplace' given an option

[azr]

This pr is based upon #230 and #231, merging it will close them; but I split them in order to allow to review the changes in smaller contextualised chunks. ( which I recommend )

This PR adds the Inplace bool field to a Request allowing to use local files 'inplace' and just setting the Operation.Dst to the source file.

This will allow Packer to not do any operation and to use big local files 'inplace'.

This PR reopens #178

[azr]

Hello @notnoop I'm sorry I reopened #178 (and source branches/PR) in order to merge them on v2, and to rebase them on the latest master, this will save us from some headaches in the future.

I picked v2 because that's what the hcl repo does: https://pkg.go.dev/github.com/hashicorp/hcl/v2?tab=doc

[jbardin]

I like the plan of putting the options into the "Request". Overall I think this will be a better approach, just left a couple comments inline.

[notnoop]

LGTM - these changes are awesome. Thanks for the redisgn. I made a couple of comments in previous PRs that carry over here, the code specific to this PR lgtm.

[azr]

Super awesome; many thanks for reviewing @jbardin and @notnoop; I'll merge this and leave the v2 branch untagged for a while in case some other breaking change needs to happen or something is wrong.