This is mainly to avoid that the toolchain property is being accessed when an EasyConfig instance is created, since that i) is time-consuming, ii) can cause problems with 'special' toolchains like the Cray* toolchains, where a craype-<optarch> module must be available when the toolchain is instantiated (which is rather silly to do when just parsing the easyconfig file).

all_dependencies is only used in specific situations, i.e. when using --dep-graph, with --job --robot, and in --check-conflicts.

The proposed change ensures that all_dependencies is correctly defined when it's first accessed.

This will help speed up parsing of easyconfig files a little bit too.