* New commandline parameter --mirror-pmOS, where the binary repository
URL for postmarketOS can be specified (empty by default as of now,
this will be filled with the real URL once the repo works)
* Do not build packages, when they are in the binary repository and
the version of the package in the binary repository is up-to-date.
* Add a testcase for pmb.build.is_necessary().
When an APKINDEX contained a package with multiple versions,
pmbootstrap did not use the last version to determine if the
package is out of date (regression from af8c9fcf5b).
Previously, it was only possible to get information about one
package inside the APKINDEX at a time.
This is needed for #64 to verify the APKINDEX.
Please note, that this implementation is actually slower, than
the previous one. But the code is more readable and it makes
caching possible (which will speed up the APKINDEX massively,
especially for the buildinfo.json file generation!)
