Allow user to pass in package URL for package reading

[LitoMore]

Related to #176

Changes

• Add importMeta option for allowing users to pass in their package URL
• Apply importMeta option to all the test code

Spec

• importMeta is required
• Its value should be import.meta

[LitoMore]

We already have a pkg option for users to pass in their package object, what about let users read the package at their side, then pass it to our package?

[sindresorhus]

I think we should accept import.meta, not import.meta.url. import.meta might get useful properties we might want to use in the future. It's more future proof this way. We can just call the option importMeta and make it required.