paths method

Meson 0.45 has a files() and and include_dirs() to deal with relative paths and current_source_dir() and project source root for absolute paths, and join_paths to join paths.

But meson does not have a paths() method that can construct a relative path similar to files which do not accept directory names.

If a parent meson.build wants to set some_base = '.' and a sub directory two levels down wants to use some_base/some_dir/mysource.c then it must know its exact location in the tree and either use ../../some_base/some_dir/mysource.c, or use some_base = join_paths(some_base, '../..') but the latter pollutes the namespace and breaks other build files.

The parent build file cannot set the path because it must be a relative path, and it doesn't want to know all details, just tell where the base is located.

A paths() function would solve this similar to how files and include_dirs work:

in parent:
some_base = paths('.')

in two levels down
my_src = join_paths(some_base, 'some_dir/my_source')

The sub build file does not need to know where it is located, it just needs the relative path to some base.

Start by tracing Meson's existing files(), include_dirs(), current_source_dir(), and join_paths() handling. Determine how a paths() API could preserve a parent-provided base for subdirectories without requiring callers to know their tree depth. Done means the behavior is specified, implemented, and covered for the parent and two-levels-down example described here.