Elastic Agent cannot be packaged without PACKAGES env var

[pchila]

Packaging agent using a command like

EXTERNAL=true SNAPSHOT=true mage package

returns an error

...
package ran for 283.017µs
Error: No packages were successfully downloaded. You may be building against an invalid or unreleased version. version=9.0.0-SNAPSHOT. If this is an unreleased version, try SNAPSHOT=true or EXTERNAL=false

It looks like the problem has been introduced by this change
https://github.com/elastic/elastic-agent/pull/6016/files#diff-520109d035a196d29d0a86e9c4e6c98e98abf056141b84df68c0d48167b87b25R1059

When no PACKAGES env var is speciified packageTypes is empty so the for loop terminates immediately.
In the rest of the packaging code and before such change an empty PACKAGES env var meant that ALL the packages should be produced.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mauri870]

Aha, that's why the command I had lying around for packaging the agent suddenly stopped working. I thought something had changed in the meantime that required specifying the PACKAGES variable manually. Now I feel bad for not investigating further.