Applies major refactoring and further implementation of
dependency resolution code in Assembler.

We are moving away from working with compiled archives
as import statements. Instead, we will be going for source code
directly. This means that all code, imported into a program,
will be loaded into a single AST and assembled in its entirety.
Removes last references to package header from language spec and
lexer. We do no need package headers in source files.