Reproduction for .mjs issues + transpile issues

This repro has a packaged vue library (sample-vue-library-0.0.5.tgz). See nuxt-community/composition-api#415 for history of repro (originally provided by @emiller12).

This repro is set up to test three different versions of the Nuxt Composition API, with pre-built & packed tarballs.

Scenario 1: .mjs + transpiling VCA

"@nuxtjs/composition-api": "file:./nuxtjs-composition-api-mjs+vca-transpile.tgz",

1. Replace version as above
2. yarn && yarn dev
3. See webpack4 error - complaining about ufo@0.6.11.

Can't import the named export 'joinURL' from non EcmaScript module (only default export is available)

Note: I removed the package exports fields from the source NCA package.json just in case this was causing the issue.

Scenario 2: .es.js + transpiling VCA

"@nuxtjs/composition-api": "file:./nuxtjs-composition-api-es+vca-transpile.tgz",

This simply renames module outputs to .es.js to avoid issue in #1 to reveal second issue.

1. Replace version as above
2. yarn && yarn dev
3. Load page twice and see the following SSR error:

ERROR  [Vue warn]: [vue-composition-api] already installed. Vue.use(VueCompositionAPI) should be called only once.

Scenario 3: .es.js + not transpiling VCA

"@nuxtjs/composition-api": "file:./nuxtjs-composition-api-es+no-vca-transpile.tgz",

1. Replace version as above
2. yarn && yarn dev

This version doesn't seem to have any issues but is here for comparison.