Simplify locating of package signatures with repod-file

repod/cli/argparse.py:
Add flag (`-s`/ `--with-signature`) for signature collection to
`repod-file package inspect` and `repod-file package import`.

repod/cli/cli.py:
Change `repod_file_package()` to more easily collect package signatures
by using `args.with_signature`.

tests/cli/test_cli.py:
Adapt tests to changes in `repod_file_package()` behavior.

repod/files/package.py:
Change `Package.from_file()` to fail if a signature file does not exist
or does not match the package name.

tests/files/test_package.py:
Adapt tests to changes in `Package.from_file()` behavior.

docs/repod/tooling.rst:
Adapt documentation to the changes in the CLI options of `repod-file
package inspect` and `repod-file package import`.
8 jobs for simplify_package_import in 12 minutes and 35 seconds (queued for 2 seconds)