Handle "pure" v1 files and projects/workspaces

[johanfylling]

When using the --v1-compatible flag with OPA, it'll behave exactly as v1.0 OPA. Meaning a Rego file isn't required to use the rego.v1 and future.keywords imports; e.g.:

package demo

p contains "Hello" if {
    input.x == 2
}

It'd be nice if Regal could be somehow instructed to deal with such files without errors or warnings.

One possible, non-obtrusive solution that would work with at least a subset of Rego projects would be to pick up the .manifest file in the workspace, and read the rego_version and file_rego_versions attributes (will be available in OPA v0.64.0).

[anderseknert]

Adding a --v1-compatible flag to regal lint seems easy enough, so let's do that. Looking for a .manifest file and use that seems like a good idea too if that's what OPA will be doing, and we can probably borrow some logic around that from OPA directly.

A PR would be welcome ;)