Install plugins command fails for maven central signatures (SHA512 checksums without a file part)

[dweiss]

Maven central artifact signatures don't carry file name in checksums. ES fails when trying to install a published plugin with such signatures (InstallPluginCommand.java).

Example of a valid plugin that currently doesn't work (checksum itself is fine):
https://repo1.maven.org/maven2/org/carrot2/elasticsearch-carrot2/7.6.0/elasticsearch-carrot2-7.6.0.zip

I don't think workarounds exist (other than republishing with just md5/ sha1). @rjernst What do you think?

[dweiss]

Just to be clear: these filename-component-less sha512 checksums are produced by Gradle and seem to be valid in the context of maven central - I see other artifacts with the same pattern (just the checksum).

I think the requirement for two components should be relaxed, here:
https://github.com/elastic/elasticsearch/blob/master/distribution/tools/plugin-cli/src/main/java/org/elasticsearch/plugins/InstallPluginCommand.java#L548-L566

otherwise it'll be hard to get maven central artifacts to work out of the box.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Plugins)