Non-root site support

[ewohnlich]

Fixes #67

[mister-roboto]

@ewohnlich thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[ewohnlich]

@jenkins-plone-org please run jobs

[mauritsvanrees]

LGTM, although I can't easily test it. It seems you cannot add a Plone Site in a sub folder in 6.1. When I create a Folder sub in the ZMI and then use the ZMI to add a Plone Site, I get a NotFound at http://localhost:8080/sub/@@plone-addsite

I suppose I could prepare this scenario in a database in Plone 6.0, migrate this to 6.1, and then try. Or indeed try something with a ZODB mount point, but it is a long time ago that I tried this.

But exporting a site in the root still works with this code, so should be fine.

One minor inline comment, but I will approve.

BTW, the same should probably be done for the export-distribution script in plone.distribution.

[ewohnlich]

FYI I meant to include a link to this post https://community.plone.org/t/adding-a-plone-site-on-a-zodb-mount-point-folder-in-6-1/22273/8 where some broader issues with folder/ZODB mount point needs are discussed - as you note, you cannot add a Plone Site outside of the root in vanilla Plone currently. We need some changes to plone.distribution for that.