[MNG-6638] - Prevent reparsing POMs in MavenMetadataSource

[oehme]

If a dependency on another project is found, don't parse its
POM again, but instead use the model we already have in memory.

This saves a large amount of time and memory for builds that have
lots of subprojects and dependencies between them.

The approach was directly copied from DefaultArtifactDescriptorReader,
which does the same optimization.

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[oehme]

Here's an allocation profile showing the severe impact this has on plugins like enforcer and assembly (jar-with-dependencies). The POM parsing overhead is marked in pink.

[eolivelli]

Very nice idea!

[slachiewicz]

The integration tests were successful:
https://builds.apache.org/view/M-R/view/Maven/job/maven-box/job/maven/job/MNG-6638/

[olamy]

LGTM

[michael-o]

@oehme Can you please rebase?

[oehme]

Done