Move test file unvendoring functionality from pyodide-build to pyodide-lock

[ryanking13]

When creating a pyodide-lock file, we separate test codes inside the wheel and pack them in a separate file to reduce the size of the wheel. This feature is highly related to the lockfile, so I think it would be nice to move it to pyodide-lock so users can use it when creating a custom lockfile using out-of-tree wheels.

On the other hand, if we move this functionality to pyodide-lock, we should support some ways to specify which packages should have their test files split and which should not. We were using unvendor_tests in the meta.yaml file, but the recipe file will not be available for out-of-tree builds.

So, probably we would need some cmdline flags such as:

--vendor-tests pkg-name pkg-name2

and also make it configurable using a config file:

[tool.pyodide.lock]
vendor-tests = ['pkg-name', 'pkg-name2']

This can also be extended to general unvendoring mechanism not only for test files as discussed in pyodide/pyodide#3092.