buildbot: weird handling of conflicting dependencies

[mojca (Mojca Miklavec)]

See ​https://build.macports.org/builders/ports-10.12_x86_64-builder/builds/17760

The port qt55-qttools fails to build because qt5-qtbase gets pulled in somehow instead of qt55-qtbase and dependencies fail to activate:

Can't install qt55-qtxmlpatterns because conflicting ports are active: qt5-qtbase

While this is probably a bug in one of qt-related ports that warrants a separate ticket, I really don't like the fact that our buildbot reports success after only checking whether packages have already been compiled:

Installing dependency (1 of 49) 'libiconv' with variants '' ... [OK]
...
Installing dependency (46 of 49) 'qt5-qtbase' with variants '' ... [OK]
Installing dependency (47 of 49) 'qt55-qtxmlpatterns' with variants '' ... [OK]
Installing dependency (48 of 49) 'qt55-qtdeclarative' with variants '' ... [OK]
Installing dependency (49 of 49) 'qt55-sqlite-plugin' with variants '' ... [OK]

Before the recently introduced changes this would report:

Installing dependency (47 of 49) 'qt55-qtxmlpatterns' with variants '' ... [FAIL]