Refactor versionguardcheck onto golang.org/x/tools/go/analysis

[michelle-clayton-work]

versionguardcheck is currently a hand-rolled Go program

Expose the analyzer as an analysis.Analyzer, replace main with singlechecker.Main, and swap our custom test code for analysistest. Adds golang.org/x/tools as the repo's first external dep.

Follow up for

• Improve versioncheck analyzer golang-fips/openssl#439