Restrict signature debug logging to vivisect backend

[acelynnzhang]

I noticed that signatures will only load when the backend is vivisect:

capa/capa/main.py

Lines 695 to 698 in 7debc54

	def get_signatures_from_cli(args, input_format: str, backend: str) -> List[Path]:
	if backend != BACKEND_VIV:
	logger.debug("skipping library code matching: only supported by the vivisect backend")
	return []

I moved the debug logging from handle_common_args to get_signatures_from_cli, so it's only printed when the signatures are actually used. This means that other backends don't need the default signatures directory to exist.

Closes #1875

Checklist

• No CHANGELOG update needed

• No new tests needed

• No documentation update needed

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[mr-tz]

see inline comment, then good to go

[acelynnzhang]

see inline comment, then good to go

is this good? thanks!

[williballenthin]

thanks!

[mr-tz]

thank you