rpk: wasm generate falsely claims that some files exist when passing an existing directory

[r-vasquez]

Version & Environment

Redpanda version: v21.11.12 (rev 937b9fe)

What went wrong?

When we create an npm template project using rpk wasm generate <project directory> and the project directory already exists, rpk will falsely claim that some files exist.

# /tmp/foo already exist

$ rpk wasm generate /tmp/foo 
unable to generate all manifest files: Files [/tmp/foo/src/main.js /tmp/foo/test/main.test.js /tmp/foo/package.json /tmp/foo/webpack.js] already exist, avoiding generation.

# neither main.js, main.test.js, package.json nor webpack.js exist

What should have happened instead?

rpk to print an error indicating that the directory already exists and a suggestion to use the command with a non-existent directory path.

rpk wasm generate --help should indicate that the argument should be a non-existent directory path

How to reproduce the issue?

1. Create an empty directory (e.g /tmp/wasmFoo)
2. run rpk wasm generate /tmp/wasmFoo