fix: Fix for java_gapic_assembly_gradle_pkg rule.

[vam-google]

More details at #3249.

The Bazel's native pkg_tar rule behaves weirdly when package_dir parameter is specified (at least on some Linux machines it does not put all the files under the package_dir).

As a workaround for that bug we tis fix creates the custom implementation of the pkg_tar rule.

[vam-google]

@aohren FYI

[codecov]

Codecov Report

Merging #3253 into master will decrease coverage by 0.00%.
The diff coverage is 44.44%.

@@             Coverage Diff              @@
##             master    #3253      +/-   ##
============================================
- Coverage     87.12%   87.11%   -0.01%     
- Complexity     6076     6078       +2     
============================================
  Files           494      494              
  Lines         24052    24058       +6     
  Branches       2613     2613              
============================================
+ Hits          20955    20958       +3     
- Misses         2234     2238       +4     
+ Partials        863      862       -1     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/api/codegen/config/DiscoApiModel.java	76.66% <0.00%> (-2.65%)	9.00 <0.00> (ø)
...google/api/codegen/config/DiscoInterfaceModel.java	57.14% <0.00%> (-2.86%)	6.00 <0.00> (ø)
...a/com/google/api/codegen/config/ProtoApiModel.java	92.30% <0.00%> (-2.43%)	18.00 <0.00> (ø)
...google/api/codegen/config/ProtoInterfaceModel.java	85.71% <0.00%> (-3.18%)	12.00 <0.00> (ø)
...om/google/api/codegen/config/ProtoMethodModel.java	88.23% <0.00%> (-1.32%)	35.00 <0.00> (ø)
...om/google/api/codegen/gapic/ProtocGapicWriter.java	69.23% <ø> (+2.56%)	5.00 <0.00> (ø)
...oogle/api/codegen/config/DiscoveryMethodModel.java	85.18% <100.00%> (+0.13%)	48.00 <1.00> (+1.00)
...a/com/google/api/codegen/util/MultiYamlReader.java	64.70% <100.00%> (ø)	5.00 <0.00> (ø)
...main/java/com/google/api/codegen/util/Scanner.java	98.79% <100.00%> (+0.01%)	37.00 <0.00> (ø)
.../java/com/google/api/codegen/discovery/Schema.java	84.84% <0.00%> (+0.50%)	43.00% <0.00%> (+1.00%)

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