Fix #271: properly resolve symlinks in all paths (#329)

I've replaced all instances in the code of `os.path.abspath`
with `os.path.realpath`, as this does the same as `abspath`
plus resolving symlinks.
See also: https://stackoverflow.com/a/40311142

test/test_challenge_build.py

@@ -21,7 +21,7 @@ import sys
 import pytest
 
 # Import from parent directory
-sys.path.append(os.path.abspath(
+sys.path.append(os.path.realpath(
     os.path.join(os.path.dirname(__file__) + "/..")))
 import pmb.build.package
 import pmb.challenge.build