Using a custom VAPI does not generate a -pkg foo flag

Nobody has claimed this yet.

The current 'Using a custom VAPI' (with pkg-config file) example on the wiki does not work for me.
I need to explicitly add a --pkg=foo to the executable line. I can only assume that this is a bug?
e.g.
change

```project('vala app', 'c', 'vala')

add_project_arguments(['--vapidir', join_paths(meson.current_source_dir(), 'vapi')],
language: 'vala')

glib_dep = dependency('glib-2.0')
gobject_dep = dependency('gobject-2.0')
foo_dep = dependency('foo') # 'foo.vapi' will be resolved in './vapi/foo.vapi'

executable('app', 'app.vala', dependencies: [glib_dep, gobject_dep, foo_dep])
```
to
```project('vala app', 'c', 'vala')

add_project_arguments(['--vapidir', join_paths(meson.current_source_dir(), 'vapi')],
language: 'vala')

glib_dep = dependency('glib-2.0')
gobject_dep = dependency('gobject-2.0')
foo_dep = dependency('foo') # 'foo.vapi' will be resolved in './vapi/foo.vapi'

executable('app', 'app.vala', dependencies: [glib_dep, gobject_dep, foo_dep], vala_args: ['--pkg=foo'])
```

BTW, on the 'Using a custom VAPI' (with no pkg-config file) example, the foo_vapi line is missing a closing parentheses.

No source file or test is named. Reproduce the two “Using a custom VAPI” wiki examples, comparing dependency handling with and without a pkg-config file; done means the generated Vala invocation handles the custom package as expected and the documentation example has the missing closing parenthesis corrected.

Written by the indexing model from the issue text.