Make webpack dirpaths relative

[kevinfodness]

Fixes a bug whereby the directory paths specified in the webpack config are relative to the calling context instead of the location of the webpack config file itself. This change allows a user to run a wp-scripts build process from somewhere other than the root directory of the plugin, such as in the root of a project (the wp-content directory), like so:

wp-scripts build --config plugins/create-wordpress-plugin/webpack.config.js --webpack-copy-php --webpack-src-dir=plugins/create-wordpress-plugin/blocks

[stevenslack]

🥳