feat: Make the configured patterns queryable

[trevorblades]

This change makes it possible to tell how the pattern option was configured for any GitRemote type. This is helpful for creating slugs for pages and knowing the basePath to pass to createFilePath. For example:

const globBase = require('glob-base');
const {createFilePath} = require('gatsby-source-filesystem');

exports.onCreateNode = ({node, getNode, actions}) => {
  const {type} = node.internal;
  if (type === 'MarkdownRemark' || type === 'Mdx') {
    const {gitRemote___NODE} = getNode(node.parent);
    const {patterns} = getNode(gitRemote___NODE);
    const {base} = globBase(patterns);
    const filePath = createFilePath({
      node,
      getNode,
      basePath: base
    });
  }
};