Use `build.ProjectBuilder.check_dependencies` for the isolated build

[agriyakhetarpal]

check_dependencies looks like a union of

1. get_requires_for_build (== dependencies for the build backend such as setuptools, meson)
2. build_system_requires (== dependencies in the pyproject.toml)

In the isolated build, we've been dealing with them separately. I guess we can use check_dependencies there too (no action required for this PR).

Originally posted by @ryanking13 in #152 (comment)