make load_one_signature use load_file_as_signatures API

[ctb]

right now it's based on signature.load_signatures which is less flexible than sourmash_args.load_file_as_signatures.

also examine load_query_signature and see if there's duplication.

[ctb]

ok, dug into this a little bit when working on #1279.

• load_one_signature is used in a variety of places for loading from either a buffer, or a string, or a file. It uses iterators internally, so will not load all the signatures in a file.
• load_query_signature is used in only a few places, and is only used for CLI functionality around loading a query signature from a file. It is not very well written.

My current hot take is that

• load_query_signature should become the public API, and it should be better written. start thinking about a standard selector framework for signature search/compatibility #1072 also seems relevant.
• load_one_signature should be our internal function.

I'm not sure how to rename them tho.

[ctb]

Huh. Working on this over in #3153, and I noticed that signature.load_one_signature is used widely in the tests, but not many other places - SBT code and sig subcommand code.

[ctb]

aaaand load_signatures is hardly used anywhere in the code base, either:

sourmash/index/__init__.py
sourmash/save_load.py
sourmash/signature.py

so we've done a great job of removing this from the core sourmash code.

(Both functions in signature.py are used all over the place in the tests, tho.)

[ctb]

This is being cleaned up a lot in #3161 which will -

• rename sourmash.signature.load_signatures to load_signatures_from_json;
• rename sourmash.signature.load_one_signature to load_one_signature_from_json;
• rename sourmash.signature.save_signatures to save_signatures_to_json;

[ctb]

This will be fixed when #3153 and #3161 are merged -

• adding sourmash_args.load_one_signature in MRG: upgrade sig overlap and sig subtract to load more than JSON signatures #3153;
• deprecating sourmash.load_one_signature and renaming current sourmash.signature.load_one_signature to load_one_signature_from_json;