Change input flag to parser

[jpreese]

This PR aims to change the --input flag to instead, be named --parser.

Reasoning:

The parser package exposes a number of different parsers, and these parsers are responsible for parsing different file formats. I feel its currently a little confusing to have conftest test --input tf, under the hood this just means to use the hcl2 parser, which we've done a mapping for. Instead, I think it makes more sense to say conftest test --parser hcl1 or conftest test --parser hcl2.

I do think theres value in being able to figure out which Parser to use, based on a file, which NewFromPath accomplishes.

This also keeps the language more consistent with what is really happening under the hood, figuring out which parser to use with the given input. As well as keeps the parser package focused on its parsers rather than specific implementations that use the language.

[Blokje5]

LGTM! It is risky to change the flag, but given that we are not at v1 yet I do not mind creating a backwards incompatible change.

As a potential option we could leave both flags in for a single release and print a deprecation warning on using the --input flag. That way users have a bit more time to adapt to the change. But i'll leave that up to you.

[jpreese]

Thanks for the review! I had the same concern as well, but being that I suspect this is a low-usage flag and being pre-v1, I didn't think it worth it to maintain backwards compatibility.