Kill Element.normal_name variable

This has been a constant source of confusion, even to me.

• For BuildStream developers, it is needlessly confusing that there are effectively 2 names of an element
• For users, there is no reason to give them inconsistent naming, just always give them the same name

The normal_name is currently used to construct artifact keys, and to create build directories.

It is also used to create log file names, these are also better off without the normal_name substitutions.

For this, it would be nice to replace the .bst with the .log extension here.