builder-module: Abort on invalid module names

[bbhtt]

This goes a step above 6d1d972 and makes it an actual error. The name is used to construct paths in various places

[swick]

The only question I have is if this will break some manifests? Or would there be errors later on anyway?

[bbhtt]

Yes looks like it errors later anyway:

flatpak-builder build --force-clean --user test.yaml
Module names like 'hell/o' containing '/' are problematic. Expect errors.
Emptying app dir 'build'
Downloading sources
Starting build of org.flatpak.hello
Cache miss, checking out last cache hit
Error: module hell/o: Error creating directory /home/bbhtt/Git/github/flatpaks/test/.flatpak-builder/build/hell/o-1: No such file or directory