Add file-backed YangToSourcesState storage

YANGTOOLS-745 deals with direct integration with BuildContext. This unfortunately does not work with plain maven, as DefaultBuildContext does not store anything and we are therefore with no state at all.

Add a separate implementation activated when we determine BuildContext does nothing and stores/restores the execution state from a single project.build.directory file. The file should follow Maven layout, i.e. should be something in target/maven-status/yang-maven-plugin/.

Using this storage we should be able to forgo generation of files across multiple maven invocations.