fix module-wide symbol visibility

[hunterjsb]

Fixes #473

Compiles workspace directory instead of single file, making functions from all files in a module visible.

Changes:

• Add workspace detection (looks for v.mod, vpkg.json, or uses file directory)
• Compile with v folder/ instead of v file.v
• Filter errors to current file
• Fallback to single-file mode when multiple main() detected

Uses existing v folder/ behavior instead of adding special VLS flags (addressing concerns from #475 and vlang/v#25703).

Tested with multi-file modules and edge cases.

[JalonSolov]

As mentioned elsewhere, blindly doing v folder/ may be just as bad as v file.v.

[hunterjsb]

closing and will wait on vlang/v#25749