search: migrate structural search logic to own file

Created by: rvantonder

I need to change how structural search interacts with indexed/zoekt search and it is becoming very tricky to work with the code in zoekt.go. After a lot of deliberation, I'm moving all structural search logic and interaction with zoekt to it's own file. This PR is a scaffolding change, there is no change in functionality. Please go through the commits to have an easy time. There are only two overarching changes:

• move currently live code in zoekt.go into it's own file search_structural.go
• wholesale duplicate the zoekt code that structural search uses and put it in search_structural.go. This is dead code.
  • simplify and rename the zoekt code that structural search uses. This is still dead code.

No tests, no live functionality changes. I will use the dead code in a follow up PR with the changes that I actually want to make.

Created by: codecov[bot]

Codecov Report

Merging #8366 into master will decrease coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8366      +/-   ##
==========================================
- Coverage   40.64%   40.59%   -0.06%     
==========================================
  Files        1290     1291       +1     
  Lines       68856    68942      +86     
  Branches     6354     6354              
==========================================
  Hits        27986    27986              
- Misses      38289    38375      +86     
  Partials     2581     2581              

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/search_structural.go	28.00% <0.00%> (ø)

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