[sdk-platform-java] maven-download-plugin is flaky when building in parallel.

[blakeli0]

The release job failed due to the following errors

00:01:33:876 [ERROR] Failed to execute goal com.googlecode.maven-download-plugin:download-maven-plugin:1.6.8:wget (download-compliance-suite) on project gapic-showcase: IO Error: OverlappingFileLockException -> [Help 1]
00:01:33:876 [ERROR] Failed to execute goal com.googlecode.maven-download-plugin:download-maven-plugin:1.6.8:wget (download-common-resources-proto) on project gapic-generator-java: IO Error: java.io.StreamCorruptedException: invalid type code: 00 -> [Help 1]

See error log for details.

download-maven-plugin:1.6.8:wget is not thread safe and could run into errors when building in parallel. We should remove the need of it in build. For this specific case, the protos we are trying to download are only used in tests, we should have a copy of them in test folders instead of always downloading from the internet.

[whowes]

@blakeli0 it looks like ${gapic-showcase.version} changes relatively frequently in gapic-showcase/pom.xml (avg. ~6 times a year), so IIUC removing usage of the plugin would require some action to make sure the checked-in version of the associated file used in tests is current each time the version gets bumped.

WDYT about adjusting the download plugin configuration everywhere it's used to have separate cache directories? I believe that would eliminate the lock contention that caused this flakiness.