cmd/llgo-dist: Don't set go/build Compiler to llgo

Without changes to go/build, this will cause an error.
Have queried golang-dev to see how this can be addressed.

Also: be less spammy with the files generated in std packages.
Miscellaneous fixes.

- If package name is "main", use as the "import path".
- Permit make(chan) without cap.
Move untyped constant conversions back into llgo

I think it'd be ideal for this to go into go/types,
but then I'd like llgo to work again...