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
For example:
```
meson builddir \
--native-file vs2019-paths.txt \
--native-file vs2019-win-x64.txt \
--cross-file vs2019-paths.txt \
--cross-file vs2019-win-arm64.txt
```
This was causing the error:
> ERROR: Multiple producers for Ninja target "/path/to/vs2019-paths.txt". Please rename your targets.
Fix it by using a set() when generating the list of regen files, and
add a test for it too.
Dylan Baker
Jussi Pakkanen
Nirbheek Chauhan
Eli Schwartz
Xavier Claessens