backport plugin bundling to 2.2

[rmuir]

This is the backport of #15233 but for maven.

It has the same filesystem layout and semantics.

The way it works in maven is, most plugin logic moves to modules/pom.xml, and plugin is a child of that, only it overrides integ logic to call install-plugin instead of install-module.

distributions for each packaging format (except integ-zip) basically just unpack modules into their modules/ directory.

The oddities/hacks/etc needed (IMO these are minor and such things are expected):

• Modules pom has a unnecessary project-level dependency on the integ-zip, that forces proper build order in the "nuclear reactor".
• Distribution pom requires distributions to "opt-in" to modules by enabling them for their specific distro. This way it itself does not depend on them and the integ-zip works, no loops, no downloading.
• Licensing works different than gradle, we inspect packaging here for the check. our module.jars are not third party dependencies: we have to generate an ignore pattern for them.
• Licensing for modules gets "duplicated": its specified in the module itself and in the distribution. We could disable it for modules, but since I think publishing artifacts for these is unavoidable with maven, and since the license-checker is packaging-based, its correct.

[nik9000]

I've added this to the top of my review.

[nik9000]

Modules pom has a unnecessary project-level dependency on the integ-zip, that forces proper build order in the "nuclear reactor".
Distribution pom requires distributions to "opt-in" to modules by enabling them for their specific distro. This way it itself does not depend on them and the integ-zip works, no loops, no downloading.
Licensing works different than gradle, we inspect packaging here for the check. our module.jars are not third party dependencies: we have to generate an ignore pattern for them.
Licensing for modules gets "duplicated": its specified in the module itself and in the distribution. We could disable it for modules, but since I think publishing artifacts for these is unavoidable with maven, and since the license-checker is packaging-based, its correct.

These are all hacks that I expected to get this working in maven.

[nik9000]

opinionated type still persists.

[nik9000]

LGTM. Thanks for the tour of the hacks.

[rmuir]

Thanks for looking this over @nik9000 !