[MySQL]Fix constraints/indexes detection bug

[k1LoW]

• When FK and UNIQUE are assigned to the same column
• When PRIMARY KEY and UNIQUE are assigned to the same column

[codecov]

Codecov Report

Merging #162 into master will decrease coverage by 1.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   68.81%   67.73%   -1.09%     
==========================================
  Files          13       13              
  Lines        2851     2851              
==========================================
- Hits         1962     1931      -31     
- Misses        692      729      +37     
+ Partials      197      191       -6

Impacted Files	Coverage Δ
drivers/mysql/mysql.go	81.99% <100%> (ø)	⬆️
datasource/datasource.go	20.85% <0%> (-19.02%)	⬇️

---

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 91afe4b...0ccde33. Read the comment docs.