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.

[codecov]

Codecov Report

Attention: Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 50.88%. Comparing base (a728506) to head (5787388).
Report is 3 commits behind head on master.

Files	Patch %	Lines
flytekit/image_spec/default_builder.py	30.00%	7 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (a728506) and HEAD (5787388). Click for more details.

HEAD has 8 uploads less than BASE

Flag	BASE (a728506)	HEAD (5787388)
	9	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2563       +/-   ##
===========================================
- Coverage   80.31%   50.88%   -29.44%     
===========================================
  Files         186      183        -3     
  Lines       18880    18736      -144     
  Branches     3915     3697      -218     
===========================================
- Hits        15164     9534     -5630     
- Misses       3059     8710     +5651     
+ Partials      657      492      -165     

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