Feature: improve logging

[erezrokah]

We have quite an intricate logic to decide what is the build directory and where to save the sitemap.xml file: https://github.com/netlify-labs/netlify-plugin-sitemap/blob/32b8aa4a2adbff23011cc5b906a9f0187ff934f2/index.js#L10

We should log which input type (env variable/plugin's input/build constant) we used and it's resolved value.