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