With the exception of things like sysconfdir (/etc), every other
installation directory option must be inside the prefix.
Also move the prefix checks to coredata.py since prefix can also be set
from inside project() with default_options and via mesonconf. Earlier
you could set prefix to a relative path that way.
This also allows us to return consistent values for get_option('xxxdir')
regardless of whether relative paths are passed or absolute paths are
passed while setting options on the command-line, via mesonconf, or via
default_options in project(). Now the returned path will *always* be
relative to the prefix.
Includes a unit test for this, and a failing test.
Closes#1299
Nirbheek Chauhan