refactor main to for ease of integration

[williballenthin]

closes #1813
closes #1946
closes #1821
closes #1945

This PR reorganizes the logic found in main() because it was getting very large and hairy. The key change is to pull out chunks of code into their own functions, typically labelled *_from_cli(), like get_backend_from_cli(). This localizes logic, which makes it easier for humans to read and make changes. It also means that main() becomes much more reasonably sized (less than 100 lines, about one screenful).

Typically we prefer to keep all input/output within main, calling library routines as necessary. This worked for a while, but main just became too large. The new "main routines" are expected to be used only within main functions, either capa main or related scripts. They reach directly into the argparse arguments, write to stdout/stderr, and can instruct the program to exit prematurely. These functions should not be invoked from library code, and this is documented.

Beyond copying code around, this PR refines our handling of the input file/format/backend. The logic for picking the format and backend is more consistent. Also, I've been careful to document that the input file is not necessarily the sample itself - cape/freeze/etc. inputs are not actually the sample.

While I was moving stuff around, I also addressed #1821, which moves a lot of potentially common routines from capa.main into the new namespace capa.loader, such as is_supported_os, get_workspace, get_extractor, etc. These routines don't depend directly on our CLI argument handling and can reasonably be used by other library code.

Here's an example of the changes needed to the freeze utility (they're basically 1 to 1):

But here's a more compelling example. The script needed to load rules and the input file, so we can replace a lot of the tedious exception handling with a consistent user interface (exceptions, status code, etc.):

TODO:

• port over script to use these new wrappers
• linters
• move routines from main into dedicated modules #1821
• rename *_from_args to *_from_cli

Checklist

• No CHANGELOG update needed

• No new tests needed

• No documentation update needed

[mr-tz]

great initiative!!

[mr-tz]

looks great so far!!

[williballenthin]

this could go into v7, though with beta1 already out, i'm not sure if its appropriate. i think these changes are solid, but if they don't pass code review easily, then we can push them off to v8.

[mr-tz]

changes look good, it's quite a big change, but I'd prefer to get this into v7 now vs. doing a breaking v8 release next week

[mr-tz]

need to figure out what's going wrong with the test_scripts (test_scripts[capa2yara.py-args0])

[williballenthin]

Tests passing again (locally, at least). I don't have any other pending changes. Merging this before v7 works for me.

[mr-tz]

LGTM, THANK YOU!

[mr-tz]

let's merge this?!