This has the adventage that "meson --help" shows a list of all commands,
making them discoverable. This also reduce the manual parsing of
arguments to the strict minimum needed for backward compatibility.
This implements support for visual studio solution directories.
Projects will by default be put into directories that map their sub-directory
name in the source folder. No directories are created if `--layout=flat` is used.
Fixes: #2524
$ flake8 | grep -E 'F841'
./run_unittests.py:1987:13: F841 local variable 'objc' is assigned to but never used
./run_unittests.py:1988:13: F841 local variable 'objcpp' is assigned to but never used
./mesonbuild/minit.py:272:5: F841 local variable 'uppercase_token' is assigned to but never used
./mesonbuild/minit.py:307:5: F841 local variable 'uppercase_token' is assigned to but never used
./mesonbuild/modules/unstable_icestorm.py:36:9: F841 local variable 'result' is assigned to but never used
./mesonbuild/modules/unstable_icestorm.py:78:9: F841 local variable 'up_target' is assigned to but never used
./mesonbuild/modules/unstable_icestorm.py:81:9: F841 local variable 'time_target' is assigned to but never used
./msi/createmsi.py:226:17: F841 local variable 'file_source' is assigned to but never used
$ flake8 | grep -E '(E123|E127|E128)'
./run_unittests.py:1358:37: E127 continuation line over-indented for visual indent
./run_unittests.py:1360:37: E127 continuation line over-indented for visual indent
./mesonbuild/minit.py:311:66: E128 continuation line under-indented for visual indent
./mesonbuild/minit.py:312:66: E128 continuation line under-indented for visual indent
./mesonbuild/minit.py:313:66: E128 continuation line under-indented for visual indent
./mesonbuild/compilers/cpp.py:115:63: E127 continuation line over-indented for visual indent
./msi/createmsi.py:156:13: E123 closing bracket does not match indentation of opening bracket's line
./msi/createmsi.py:188:13: E123 closing bracket does not match indentation of opening bracket's line