support standalone manifests containing zip files

[ctb]

build a manifest containing zip files with sig collect:

sourmash sig collect podar-ref/1.fa.sig.zip -o manifest-with-zipfiles.csv -F csv

try to run manysearch on it:

sourmash scripts manysearch manifest-with-zipfiles.csv manifest-with-zipfiles.csv -o xxx.csv

and you will get:

Reading query(s) from: 'manifest-with-zipfiles.csv'
Loaded 1 query signature(s)
thread '<unnamed>' panicked at src/manysearch.rs:32:90:
called `Result::unwrap()` on an `Err` value: Error: Failed to load query record: CP001941.1 Aciduliprofundum boonei T469, complete genome
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "/Users/t/miniforge3/envs/py311/bin/sourmash", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/t/dev/sourmash/src/sourmash/__main__.py", line 20, in main
    retval = mainmethod(args)
             ^^^^^^^^^^^^^^^^
  File "/Users/t/dev/pyo3_branchwater/src/python/sourmash_plugin_branchwater/__init__.py", line 71, in main
    status = sourmash_plugin_branchwater.do_manysearch(args.query_paths,
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pyo3_runtime.PanicException: called `Result::unwrap()` on an `Err` value: Error: Failed to load query record: CP001941.1 Aciduliprofundum boonei T469, complete genome

Also note that we should load files from within manifests as if they are relative to the manifest dir per sourmash-bio/sourmash#3054 and sourmash-bio/sourmash#3008 (comment).

Originally noted in #237.