fix(gatsby-source-filesystem): dereference symlinks when copying files with publicURL

[jgosmann]

Description

I have some larger assets in one of my Gatsby projects that I pull in with gatsby-source-filesystem. Due to the file sizes involved, I want to avoid committing those files to git. Instead I'm using git-annex which replaces the files with relative symbolic links (e.g. ../../../../.git/annex/objects/.... Currently, gatsby-source-filesystem copies the link itself to the public folder when building the site which breaks the link.

This PR adds the "dereference" option to the copy call which will cause the actual file content to be copied and produce a working build even if symbolic links are involved.

Documentation

As far as I am aware it is not documented how symlinks are handled. Not sure if it needs to be, I would expect symlinks to be dereferenced anyways.

Related Issues

None that I am aware of.

[pieh]

Thanks @jgosmann!

Tested and verified it works correctly.

I expect we might need similar changes in few other places as well, but this is good to go in on its own (no point delaying the fix)

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