feat: improve codegen compat, use var __component__ instead of var component

[IndexXuan]

for userland code, If code like this, there will be variable conflicts.

import component from './HelloWorld2.vue'

component.name = 'HelloWorld2'

export default component

the genereated code will have var component which conflicts with user-defined vars.

this PR use __component__ for better compat with userland code.

[IndexXuan]

@underfin plz review it. thank you.