[MRESOLVER-570] Remove excessive strictness of OSGi dependency metadata

[HannesWell]

This is a forward port of #505 to resolver-2.x/master.

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• Mark optional Maven dependencies as optional in OSGi metadata
• Widen version range for org.slf4j.spi package to [1.7,3)
• Move bnd configuration section from execution to plugin configuration in order to simplify extensions in sub-modules.

Fixes MRESOLVER-570

[gnodet]

@HannesWell Same comment as #505 (comment)
I wonder if we should lower the import range to [2.0,2.1) for intra project imports... ?

[gnodet]

@HannesWell Same comment as #505 (comment)
I wonder if we should lower the import range to [2.0,2.1) for intra project imports... ?

[HannesWell]

For resolver-2x this change is actually simpler than for 1.9.x since maven-resolver-impl does not reference Sisu, Guice and org.slf4j.spi in its code anymore. The classes Slf4jLoggerFactory and AetherModule have been dropped for resolver-2.0.

[gnodet]

LGTM, however, optional imports should be minimised when possible.

[HannesWell]

Thank you for your review and submitting this.

LGTM, however, optional imports should be minimised when possible.

In general yes. But most of the time problems with it arise if the code is not really prepared to handle the absence of a bundle, since this is not that easy. But since the Maven dependencies are also marked as optional and I assume the code is prepared to handle the absence correctly respectively the use-cases are designed accordingly I think it should be fine.