Xlang Dockerfile speed improvements

[Polber]

Add same speed improvements that were added to YamlTemplate in #1405. This will greatly reduce startup time, remove need to specify requirements.txt in every module that uses Python UDF's (using root pom.xml for versioning beam), and is one step closer to consolidating Yaml Dockerfile and Xlang Dockerfile (will need another change on Beam before merging).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.67925% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 40.07%. Comparing base (f696a2d) to head (592c3e1). Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1499      +/-   ##
============================================
+ Coverage     39.97%   40.07%   +0.09%     
+ Complexity     2806     2803       -3     
============================================
  Files           753      754       +1     
  Lines         42697    42700       +3     
  Branches       4581     4581              
============================================
+ Hits          17070    17112      +42     
+ Misses        24150    24110      -40     
- Partials       1477     1478       +1     

Components	Coverage Δ
spanner-templates	56.71% <ø> (-0.02%)	:arrow_down:
spanner-import-export	65.57% <ø> (-0.05%)	:arrow_down:
spanner-live-forward-migration	61.23% <ø> (ø)
spanner-live-reverse-replication	42.53% <ø> (ø)
spanner-bulk-migration	72.00% <ø> (ø)

Files	Coverage Δ
...oud/teleport/plugin/PythonDockerfileGenerator.java	85.71% <ø> (+60.71%)	:arrow_up:
...loud/teleport/plugin/XlangDockerfileGenerator.java	88.67% <88.67%> (ø)

... and 3 files with indirect coverage changes