fix(gatsby-source-filesystem): Support creating file nodes for paths containing folders

[jshearer]

Description

I encountered a subtile bug in the logic for handling createFileNodeFromBuffer. My intention is to create file nodes nested inside folders. When I try this currently (without this PR), the containing folder hierarchy is not created, since the call to fs.ensureDir is passed a different directory than writeBuffer. This PR fixes that.

Documentation

The docs for createFileNodeFromBuffer are here.

Related Issues

There are no related issues that I could find.

[pieh]

Yup, makes sense. Thanks for handling it!

[gatsbot]

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