Allowing custom targets to depend on builtin targets

[mitchgrout]

A use case I have is to instrument my unit tests to emit coverage information, run all of the tests, and then use gcovr or similar to collate the generated results. It would be convenient if I were able to write something like...

test = builtin_target('test')
target = custom_target(
  depends: builtin_target('test'),
  command: [gcovr, ...],
  output: ['coverage.txt', 'coverage.xml']
)
alias_target('coverage', target)

...as then I can do the following to generate my coverage reports:

$ meson setup build
$ ninja -C build coverage
# ...runs unit tests
# ...runs gcovr
# ...spits out coverage.txt

Unfortunately there's no way to currently specify this relation between my custom_target and a builtin like test, so I instead have to have this odd loose coupling when running the task

$ meson setup build
$ ninja -C build test
# ...runs unit tests
$ ninja -C build coverage
# ...runs gcovr
# ...spits out coverage.txt

and if I forget to run test before (or have stale results), then the coverage output is going to be incorrect.

Being able to fetch one of the builtin targets via a method like builtin_target(str) would allow me to correctly sequence this task. There are some drawbacks I can see:

1. What if you specify a new test() after declaring this target that depends on all tests? Could be an error similar to how add_project_arguments fails after defining the first build target
2. What if you allow builtin_task('all') and build a cycle? Ninja would detect, but certain tasks could be excluded

Thoughts?