fix: dont assume plugin-help is installed

[mdonnalley]

Use Help class directly instead of assuming that plugin-help is installed.

QA

• pull branch, yarn && yarn build
• oclif generate my-cli
• Remove all references to plugin-help from my-cli's package.json
• Remove src/hello/index.ts
• yarn link @oclif/core in my-cli directly
• Run bin/dev.js hello - you should see help output for the hello topic

Fixes #848
@W-15103728@

[shetzel]

QA:
Followed the steps above and saw the expected output.

bin/dev.js hello
Say hello to the world and others

USAGE
  $ my-cli hello COMMAND

COMMANDS
  hello world  Say hello world