fix(compiler): convert local templates to an engine field for nested templates

[ecrupper]

When leveraging vela validate pipeline --template --template-file exampleOne:one.yml,exampleTwo:two.yml in a nested template sense (exampleTwo is called within exampleOne), the current implementation of local template retrieval can't handle that.

By adding local templates to the engine itself, we can access the provided info at any time during compilation, allowing proper local retrieval deep within a template chain.

[codecov]

Codecov Report

Merging #916 (e219b5e) into main (41fdfd1) will increase coverage by 0.10%.
The diff coverage is 34.61%.

@@            Coverage Diff             @@
##             main     #916      +/-   ##
==========================================
+ Coverage   71.37%   71.47%   +0.10%     
==========================================
  Files         304      304              
  Lines       12466    12454      -12     
==========================================
+ Hits         8897     8901       +4     
+ Misses       3134     3120      -14     
+ Partials      435      433       -2     

Files Changed	Coverage Δ
compiler/native/expand.go	71.23% <0.00%> (-2.96%)	⬇️
compiler/native/compile.go	67.91% <100.00%> (+4.25%)	⬆️
compiler/native/native.go	86.27% <100.00%> (+0.56%)	⬆️

[KellyMerrick]

lgtm