<extends> dependency should be resoulved in dist:build-src

along the lines of MPARTIFACT-49, if the pom for a project extends another pom, the released pom for the project should resolve the extension (i.e. the pom which is extended should be included into the original POM).
For example, if one has <extends>../project-base.pom</extends>, the extension cannot be resolved in the source distribution of the project, and so the project cannot be built from the source distribution.