Publish some property with the IU to support automatic filtering

[laeubi]

I'm currently think about a feature where Tycho automatically select a JusJ:

• Support automatic installation of a suitable JustJ in a product packaging eclipse-tycho/tycho#2064

Tycho can already filter quite good what will suite a product, but some things must be supplied by the user:

1. jdk or jre
2. stripped debug symbols or not

it would therefore be good if the IUs of a JRE can either:

• define some properties (e.g. jre.type = jdk, jre.debugSymbols=true)
• define a capability (e.g. java.jdk, java.jre or java.debug)
• ... some other way that could be quiried / filtered in p2 ...

[merks]

I'm currently think about a feature where Tycho automatically select a JusJ:

* [Support automatic installation of a suitable JustJ in a product packaging eclipse-tycho/tycho#2064](https://github.com/eclipse-tycho/tycho/issues/2064)

Tycho can already filter quite good what will suite a product, but some things must be supplied by the user:

1. jdk or jre

JustJ doesn't provide JDKs.

2. stripped debug symbols or not

You can query for the iu with .stripped appended.

it would therefore be good if the IUs of a JRE can either:

* define some properties (e.g. jre.type = jdk, jre.debugSymbols=true)

* define a capability (e.g. `java.jdk`,  `java.jre` or `java.debug`)

* ... some other way that could be quiried / filtered in p2 ...

I think there is already enough information to build an IU ID which can be used as the basis for a query:

org.eclipse.justj.openjdk.hotspot.jre.(full|minimal|base)(.stripped)?

It seems to me that the setup for this is already very minimal, just using standard existing things:

The "confusing" steps in the documentation is to a large extent because one has to do it several different ways depending on which version of Tycho is involved.

In any case, I expect that what's available via the current simple naming conventions ought to be sufficient.

[merks]

I don't know that anything concrete beyond what's already available is needed.

[laeubi]

@merks I finally was able to actually implement the feature that would using it here:

• Add support for including a JRE in the product eclipse-tycho/tycho#3816

As you can see Tycho creates a requirement to JustJ JRE here

(MetadataFactory.createRequirement(TychoConstants.NAMESPACE_JUSTJ, jreName, null, null, false, false)

jreName is currently jre but would be good to allow being more specific, for example if I wanted always a stripped JRE I probably like to use jre.stripped or if I always want a full JRE then jre.full.

Similar might b e useful for the minimal versus base so in the end each JRE would just provide some additional capabilities to better control this.

[laeubi]

From my limited understanding something like this is what I want:

• Provide additional capability that includes the jvmName justj.tools#12

that makes it possible to create a query in P2 that filters for a specific JustJ variant, e.g. like

providedCapabilities.exists(p | p.namespace == 'org.eclipse.justj' && p.name == 'jre.full')