Add gitea-vet

[jolheiser]

@lunny I'm not sure whether this can be incorporated into the build folder because of how go vet wants it to work, let me know if I need to change something.

This PR adds gitea-vet as a custom vet tool.

Currently it only has two checks, more can be added later.

1. Check for sort order. stdlib -> code.gitea.io -> other
2. Check file headers for some form of Copyright...YYYY...Gitea/Gogs

[lunny]

Pelase resolve the conflicts.

[jolheiser]

Don't merge this yet. I have a few things to clean up since #10964 was merged.

[jolheiser]

Okay, so a few more things in here.

I combined tools.go into build.go since they effectively did the exact same thing.
As well, I removed the extra tag from our Makefile.

go mod tidy:

Prune any no-longer-needed dependencies from go.mod and 
add any dependencies needed for other combinations of OS, architecture, and build tags

So we will still vendor them without explicitly giving a build tag.

[codecov-io]

Codecov Report

Merging #10948 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10948      +/-   ##
==========================================
- Coverage   43.62%   43.61%   -0.02%     
==========================================
  Files         597      597              
  Lines       83923    83923              
==========================================
- Hits        36613    36599      -14     
- Misses      42799    42811      +12     
- Partials     4511     4513       +2     

Impacted Files	Coverage Δ
models/lfs.go	17.27% <ø> (ø)
modules/lfs/content_store.go	45.33% <ø> (ø)
modules/lfs/server.go	40.35% <ø> (ø)
routers/api/v1/misc/signing.go	0.00% <ø> (ø)
routers/repo/middlewares.go	59.09% <ø> (ø)
routers/swagger_json.go	0.00% <ø> (ø)
services/pull/check.go	48.17% <0.00%> (-3.66%)	⬇️
modules/process/manager.go	74.69% <0.00%> (-3.62%)	⬇️
services/pull/patch.go	62.93% <0.00%> (-2.80%)	⬇️
services/pull/temp_repo.go	29.05% <0.00%> (-2.57%)	⬇️
... and 3 more

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