Merge #5951 #5975

Merge #5951 #5975

5951: Rename record_field_pat to record_pat_field r=jonas-schievink a=pksunkara

The token was renamed but not this.

5975: Report better errors in project.json/sysroot r=jonas-schievink a=jonas-schievink

This does a bunch of light refactoring so that the `Sysroot` is loaded later, which makes sure that any errors are reported to the user. I then added a check that reports an error if libcore is missing in the loaded sysroot. Since a sysroot without libcore is very useless, this indicates a configuration error.

Co-authored-by: Pavan Kumar Sunkara <pavan.sss1991@gmail.com>
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>