MRG: prepare to remove sourmash compute for sourmash v5.0 (#3103)

This PR refactors code to eliminate any internal dependencies on the
`compute` command or codebase, in preparation for removing `sourmash
compute` in v5.0, per
#1286.

Specifically, this PR:
* shifts common sketching code from `src/sourmash/command_compute.py` to
`src/sourmash/command_sketch.py`;
* refactors three tests that were still using `sourmash compute` to use
`sourmash sketch` instead.

No functionality is altered or adjusted in this PR; it's all just code
refactoring.

The next step for #1286 would maybe be adjust the code and the tests to
respect the `--v4` and `--v5` flags (as used in
#3072 /
#3074). The actual compute
code and command wouldn't be removed until after a 5.0 release, I think.

src/sourmash/command_compute.py

@@ -13,9 +13,15 @@
 from .utils import RustObject
 from ._lowlevel import ffi, lib
 
-DEFAULT_COMPUTE_K = "21,31,51"
-DEFAULT_MMHASH_SEED = 42
-DEFAULT_LINE_COUNT = 1500
+
+from .command_sketch import (
+    _compute_individual,
+    _compute_merged,
+    ComputeParameters,
+    add_seq,
+    set_sig_name,
+    DEFAULT_MMHASH_SEED,
+)
 
 
 def compute(args):