Clean-up lookup functions in semanal.py

[ilevkivskyi]

Second pass of semantic analysis has 6 lookup functions:

• lookup
• lookup_qualified
• lookup_fully_qualified
• lookup_fully_qualified_or_none
• lookup_type_node

plus 3 related helpers:

• builtin_type
• named_type
• named_type_or_none

I think 9 lookup functions is too much and can cause confusions (this happened to me several times). I think we can leave only 3 lookup functions plus 2 helpers.

[ilevkivskyi]

I think we need to fix this soon. Already three lookup functions and two lookup helpers sneaked in the plugin API (which is I believe too many).

[ilevkivskyi]

(Added the new analyzer label. Although it is not specific to the new analyzer, it is a great opportunity to clean things up.)

[97littleleaf11]

There also exist several lookup funcs in checker.py. I just have a glanced at them and it seems they share the same logic with those in other files. Shall we extract them to lookup.py?

[ilevkivskyi]

Yeah, I think moving everything to one place should be good.