Fall back to Go module build info when version isn't set via ldflags

[stickpro]

Summary

• Version/Commit (added in feat: Add version and commit information to easyjson generator #424) are only populated via -ldflags at release build time (see Makefile). go install github.com/mailru/easyjson/easyjson@v0.9.2 doesn't pass those flags, so the installed binary always reports version: dev / commit: none.
• versionInfo() now falls back to runtime/debug.ReadBuildInfo() when Version/Commit are left at their defaults, picking up the module version and VCS revision that the Go toolchain embeds automatically for both go install pkg@version and local go build in a git checkout. ldflags-set values (used by the release Makefile) still take priority.

Closes #438