Reworked ParentGatherer and related pom.xml manipulations

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
Fixed missing error marker when builder can't read pom.xml

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
updated to match corresponding m2e-core changes

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
added test for facade.getMojoExecutions without maven context

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
moved facade/project tracking out of MavenExecutionContext

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>