Allow for flytekit version to be specified in default image builder

[thomasjpfan]

Why are the changes needed?

With 1.13.0, the default image builder does not allow flytekit to be specified to a specific version. For example, the following will fail:

from flytekit import ImageSpec, task

image1 = ImageSpec(
    packages=["numpy==1.26.4", "flytekit==1.12.3"],
    builder="default",
)


@task(container_image=image1)
def work():
    pass

 pyflyte build build_flyte.py work

with:

0.220   × No solution found when resolving dependencies:
0.221   ╰─▶ Because you require flytekit==1.13.0 and flytekit==1.12.3, we can
0.221       conclude that the requirements are unsatisfiable.

What changes were proposed in this pull request?

With this PR, we check for flytekit first. If it is not detected in requirements, then we inject flytekit into the requirements with the version used during registration.

How was this patch tested?

Unit test was added to this PR.