use re.fullmatch on instance "name" attribute which is supposedly unique
use re.fullmatch on instance "name" attribute which is supposedly unique

- mutatorMath readInstance method stops at the first occurrence, so it is
better to use a more unique key to search like the "name" attribute
instead of the "stylename" which might be shared with other instances.
In glyphsLib's generated designspaces, the instance "name" is by default
the full name, i.e. family name + style name, e.g. "Noto Sans Regular").

- we raise FontmakeError if no match is found.

- factor out os.path.normcase(os.path.normpath(...)) into _normpath helper

googlefonts/fontmake,googlei18n/fontmake,googlei18n/fontmake,googlefonts/fontmake