refactor: improve build

[caarlos0]

prepare the build api for future features:

• move matrix gen to another pkg
• merge target stuff with build opts
• renamed builds.lang to builds.builder

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/goreleaser/goreleaser/3tyUBGSM3HCtCGA7RoLBFM5FsJ1v
✅ Preview: https://goreleaser-git-build-goreleaser.vercel.app

[codecov]

Codecov Report

Merging #2475 (33ed61f) into master (8241a23) will decrease coverage by 0.29%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master    #2475      +/-   ##
==========================================
- Coverage   84.49%   84.19%   -0.30%     
==========================================
  Files          88       88              
  Lines        6340     6335       -5     
==========================================
- Hits         5357     5334      -23     
- Misses        827      840      +13     
- Partials      156      161       +5     

Impacted Files	Coverage Δ
pkg/config/config.go	90.72% <ø> (ø)
internal/builders/buildtarget/targets.go	94.69% <50.00%> (ø)
internal/pipe/build/build.go	88.77% <86.95%> (-9.53%)	⬇️
internal/builders/golang/build.go	92.14% <100.00%> (+0.51%)	⬆️
internal/tmpl/tmpl.go	95.83% <100.00%> (+0.07%)	⬆️
pkg/build/build.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8241a23...33ed61f. Read the comment docs.