this should help users specify leading `~` in various Meson options and variables
without refactoring lots of places inside Meson itself.

fs: make exception specify method name

fs: actually raise exceptions

fs: resolve path e.g.  /opt/foo/.. => /opt/foo

fs: correct behavior of is_symlink

is_samepath better reflects the nature of this function--that files
and directories can be compared.

Also, instead of raising exceptions, simply return False when one
or both .is_samepath(path1, path1) don't exist. This is more
intuitive behavior and avoids having an extra if fs.exist() to go
with every fs.is_samepath()