This moves coverage.h to src/coverage/, and specifies path to coverage.h
with files() directive, to make it can be included even when located
outside of the include directories. Otherwise, libc-wrapper cannot be
built when -Db_coverage=true option is enabled.
Follow-up for 543a48b653.
meson by default adds the current source and build directory as include
directories. Because we structure our meson code by gathering a giant dict
of everything we want to do and then doing all the actual target generation
in the top level meson.build, this behavior does not make sense at all because
we end up adding the top level repository directory as an include directory
which is never what we want.
At the same time, let's also make sure the top level directory of the build
directory is not an include directory, by moving the version.h generation
into the src/version subdirectory and then adding the src/version subdirectory
of the build directory as an include directory instead of the top level
repository directory.
Making this change means that language servers such as clangd can't get
confused when they automatically insert an #include line and insert
"#include "src/basic/fs-util.h" instead of "#include "fs-util.h".
Let's split off a new vcs-tag option from version-tag that configures whether
the current commit should be appended to the version tag. Doing this saves
us from having to fiddle around with generating git versions in packaging
specs and instead let's meson do it for us, even if we pass in a custom
version tag.
With this approach there's no more need for tools/meson-vcs-tag.sh so
we remove it.
