fix: deprecated aliases bug

[mdonnalley]

Fix bug with deprecateAliases warning showing when flag name starts with deprecated alias.

QA

• oclif generate my-cli
• add this flag definition to src/commands/hello/index.ts

    'foo-bar': Flags.boolean({
      summary: 'summary',
      deprecateAliases: true,
      aliases: ['foo']
    }),

• checkout this branch and run yarn build
• yarn link @oclif/core in my-cli directory
• bin/dev.js hello oclif --from me --foo-bar should not emit warning
• bin/dev.js hello oclif --from me --foo should emit warning

Fixes #1204
@W-16866149@

[soridalac]

the changes looks good!