Issue #492 restore compile-only reactor module so downstream resoluti…

[yeroc]

Fixes #492

Description

restoreProjectArtifacts only wires up project.getArtifact() to a restored artifact when the cached file is a regular jar. For compile-only cached builds the primary cached artifact is the target/classes directory, which falls through that condition unset. In a multi-module reactor that leaves downstream modules unable to resolve the upstream as a reactor artifact on a subsequent mvn clean compile, and they fall through to remote repositories with Could not find artifact ...:jar:...-SNAPSHOT.

For the directory case this PR sets the file on the existing project artifact instead of replacing it with a RestoredArtifact. RestoredArtifact.getFile routes through restoreToDiskConsumer on every call, which would later break package-phase plugins (e.g. jar:jar) that update the file via setFile.

Added CompileOnlyReactorRestoreTest, which reuses the existing issue-393-compile-restore reactor project to exercise two consecutive compile-only invocations. The test fails on master and passes with this change.

Checklist

• Your pull request should address just one issue, without pulling in other changes.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied.
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• You have run the integration tests successfully (mvn -Prun-its verify).
• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

[yeroc]

I considered renaming the existing issue-393-compile-restore reactor project to something more generic since it covers a fairly common and important case with multi-module reactor and internal dependencies that isn't covered elsewhere as far as I can tell. Thoughts on renaming that?

[yeroc]

@sparsick looking at the failure for build # 2 it looked like a strange intermittent issue pulling down transitive dependencies from Maven Central. The 3rd build was cancelled. As far as I can tell the failure doesn't appear to be related to the changes I made? Thoughts?