Registry tool improvements

Building on changes previewed in v0.5.9:

registry get now lists resources and replaces registry list, which is removed. When the -o yaml option is provided, registry get returns resources in a YAML format. When PATTERN matches multiple objects, a YAML list is returned. When the -o contents option is specified, contents of spec and artifact resources are written to stdout.
registry export PATTERN can be used to write subsets or the entire contents of a registry to a local directory of YAML files.
Both registry get and registry export accept a --filter argument.
registry apply can be used to apply all YAML files returned by registry get (including lists) and registry export to the registry.

Several obsoleted commands have been removed:

registry upload manifest, registry upload manifest, and registry upload manifest are all replaced by registry apply.
registry upload spec is removed, replaced by registry apply.

Other command changes include:

The bulk uploaders ('openapi', 'discovery', 'protos') under registry upload bulk are now directly under registry upload.
registry export csv is moved to the registry-experimental tool in https://github.com/registry-experimental.

Test coverage is above 60% (and climbing!)

Changelog

1b8f3b2 Extend registry export to take patterns and --filter arguments. (#942)
665921c Move files only needed for registry compute commands out of core (#943)
94c5460 Add tests for names package. (#940)
d4de027 Add support for exporting, getting, and applying receipts in YAML. (#941)