Sapper Export with basepath results in writing outside the build directory.

[antony]

Describe the bug
An extension of #502

To Reproduce
[ ] Run export with a basepath with a leading slash

An export directory is created in the root of your project, rather than __sapper__/export. Whilst this is preferable to the original behaviour in #502 - it's still wrong, and could cause chaos.

Expected behavior
Running export with basepath with a leading slash should build your exported app at __sapper__/export/<basepath>.

Severity
Not super serious, but could cause unwanted side-effects, especially if your basepath was src or something equally silly.

[antony]

After a lot of investigation, this appears to have been a nuance of the way that a third party had set up a template, and I'm unable to recreate it with a new project.