fix: pass file pathes to `postcss.process()` (#77)
fix: pass file pathes to `postcss.process()` (#77)

Motivation: In certain use cases, autoprefixer and browserslist ended up looking up an `undefined` path since postcss explicitly sets plugin options, thus overwriting a file path to `undefined`.

[debug] processAssets ng-packaged/my-lib to ng-packaged/.ng_build/ts
[debug] render stylesheet ng-packaged/my-lib/src/foo/foo.component.scss
[debug] rendering sass for ng-packaged/my-lib/src/foo/foo.component.scss
[debug] postcss with autoprefixer for ng-packaged/my-lib/src/foo/foo.component.scss
BUILD ERROR
Path must be a string. Received undefined
TypeError: Path must be a string. Received undefined
at assertPath (path.js:7:11)
at Object.resolve (path.js:1146:7)
at eachParent (ng-packaged/node_modules/ng-packagr/node_modules/browserslist/index.js:65:18)
at getStat (ng-packaged/node_modules/ng-packagr/node_modules/browserslist/index.js:79:12)
at browserslist (ng-packaged/node_modules/ng-packagr/node_modules/browserslist/index.js:188:15)
at Browsers.parse (ng-packaged/node_modules/ng-packagr/node_modules/autoprefixer/lib/browsers.js:61:16)
at new Browsers (ng-packaged/node_modules/ng-packagr/node_modules/autoprefixer/lib/browsers.js:52:30)
at loadPrefixes (ng-packaged/node_modules/ng-packagr/node_modules/autoprefixer/lib/autoprefixer.js:67:24)
at plugin (ng-packaged/node_modules/ng-packagr/node_modules/autoprefixer/lib/autoprefixer.js:78:24)
at LazyResult.run (ng-packaged/node_modules/postcss/lib/lazy-result.js:270:20)
dherges/ng-packagr,dherges/ng-packagr,dherges/ng-packagr,dherges/ng-packagr,DavidParks8/ng-packagr,DavidParks8/ng-packagr,DavidParks8/ng-packagr,DavidParks8/ng-packagr