tinygo: revise and simplify wasmtime argument handling

[ydnar]

This PR implements 3 related changes to how TinyGo processes arguments for the wasmtime emulator. I ran across the issue related to trying to use wasmtime serve in a custom target file for my WasmCon talk next month.

It adds support for alternative wasmtime subcommands like serve, simplifies the Wasmtime-specific code paths, and fixes PWD handling for tinygo run when not running tests. I recommend reviewing by commit to see the individual changes.

Detailed changes:

• Simplify buildAndRun, condensing wasmtime handling into a single section handling --env and --dir args.
• Use wasmtime run subcommand instead of bare wasmtime. This enables use of wasmtime serve or other subcommands. The key change is that emulator arguments are inserted after the subcommand, e.g. run.
• Set PWD to the package dir only when running tests, which allows tinygo run --target wasip1 to run with PWD=..
• Fixed a related bug with -x (PrintCommands) where it repeated the full path and name of the command.

[dgryski]

The isSingleFile logic was for cases where we did tinygo run some/dir/file.go to make sure the directories were set correctly. If that still works then these cleanups LGTM.

[ydnar]

Yes that still works.

The equivalent tinygo run ./cmd/foo also works now (without specifying main.go).