Task semantics behaviour

[zuzuleinen]

This PR implemented all scenarios from #229 under Behavior based on those changes section.

More specific:

• input is mandatory on build tasks except when task has rebuild:always or is a compound task(has only dependsOn node on it)
• target and rebuild: always are not allowed on the same task. They are incompatible now.
• a new state was added StateNoRebuildRequired which is similar StateCached. The differennce is that the reason for StateCached is because the targed was fetched from cache while the reason for StateNoRebuildRequired was that a rebuild was not necessary but there was no target to fetch from cache.

Tests for functionality are added in test/e2e/tasksemantics. The are all green, however there are still tests that needs fixing.

[Equanox]

• The task status no-rebuild feels not right. E.g. npm run lint or go fmt doesn't build anything.
• Shall we avoid showing a state on "compound" tasks? It's a bit useless.
• I see some difficulties during testing when forbidding rebuild: always together with target: (aka redundant targets). Shall we make the behavior adjustable? Adding a mode on which we allow Bob redundant targets, WithAllowRedundantTarget()