Refactor: clean-up cmd filenames

[apostasie]

Following-on previous cmd cleanups, this one shorten / rename the files.

Anything that stutters is reduced:
cmd/nerdctl/category/category_(.+).go > cmd/nerdctl/category/$1.go

Top-level files are renamed to command.go and command_test.go.

[apostasie]

At 58 minutes+ the EL8 tests are presumably stuck.

I believe we can ignore for this specific PR - and see if I can get somewhere fixing it in #3965

[AkihiroSuda]

Sorry, I'd prefer the file names to be as unique as possible

[apostasie]

Sorry, I'd prefer the file names to be as unique as possible

This is the opposite of what is currently done in pkg/cmd. eg: pkg/cmd/container/commit.go.

Are you saying you would prefer them to be pkg/cmd/package/package_x.go then?

[AkihiroSuda]

Are you saying you would prefer them to be pkg/cmd/package/package_x.go then?

That would be preferable when there is another x.go

[apostasie]

Are you saying you would prefer them to be pkg/cmd/package/package_x.go then?

That would be preferable when there is another x.go

I see.