For uncached workspaced elements, assemble is required even just to
calculate the cache key. As dynamic scheduling relies on cache keys,
schedule assemble for uncached workspace elements even if they have not
been marked as required yet.

Fixes #461.

Valentin David authored Aug 15, 2018 and Jürg Billeter committed Aug 28, 2018

This adds a regression test for #461.

Fix key invalidation for workspaced dependencies

Closes #461

See merge request !740

Tristan Van Berkom authored Aug 28, 2018 and Jürg Billeter committed Aug 28, 2018

Removed redundant tests from recently merged !740, this new
test does not need to run for every different source kind.

tests/frontend/workspaces.py: Removing some redundant tests

See merge request !741

This aims to address #600, this will raise an exception when a resolved
variable contains a reference to the variable.