The `AstVisitor` intentionally ignores whitespaces and symbols,
as they are not useful for tne interpreter. However, when formatting a
build file, we need them. This commit introduces a `FullAstVisitor` that
visits every Nodes, including whitespaces and symbols.
This replaces all of the Apache blurbs at the start of each file with an
`# SPDX-License-Identifier: Apache-2.0` string. It also fixes existing
uses to be consistent in capitalization, and to be placed above any
copyright notices.
This removes nearly 3000 lines of boilerplate from the project (only
python files), which no developer cares to look at.
SPDX is in common use, particularly in the Linux kernel, and is the
recommended format for Meson's own `project(license: )` field
Charles Brunet