searcher: factor client out of graphlbackend

Created by: keegancsmith

This doesn't improve the client or change it at all. It just cuts and pastes it into its own package.

We now will "copy" the data. Previously we intentionally unmarshalled straight into the resolvers to be more efficient. However, we never do large searches with searcher anymore. Additionally Zoekt does large scale searches and also does a copy.

This change moves us towards a potentially more general interface that all search clients could implement.

Created by: codecov[bot]

Codecov Report

Merging #13131 into main will increase coverage by 0.04%. The diff coverage is 7.69%.

@@            Coverage Diff             @@
##             main   #13131      +/-   ##
==========================================
+ Coverage   51.14%   51.19%   +0.04%     
==========================================
  Files        1470     1470              
  Lines       82533    82444      -89     
  Branches     6704     6704              
==========================================
- Hits        42215    42210       -5     
+ Misses      36712    36627      -85     
- Partials     3606     3607       +1     

Flag	Coverage Δ
#go	52.48% <7.69%> (+0.06%)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/textsearch.go	64.68% <7.69%> (+12.81%)
internal/db/helpers.go	100.00% <0.00%> (ø)
internal/gosrc/stdlib.go	100.00% <0.00%> (ø)
internal/conf/platform.go	100.00% <0.00%> (ø)
internal/routevar/util.go	100.00% <0.00%> (ø)
internal/timeutil/week.go	100.00% <0.00%> (ø)
internal/vcs/git/mocks.go	100.00% <0.00%> (ø)
internal/db/query/query.go	100.00% <0.00%> (ø)
internal/highlight/mocks.go	100.00% <0.00%> (ø)
... and 732 more