perf: Optimize templates resource loading peformance

[filzrev]

What's included in this PR
Small perf optimization related to templates resource loading.

• Remove unnecessary CreateTemplateResource() call.
• Call ToArray() before IResourceFileReader::Names property is set.
  Because Names are enumerated multiple times.
• Add HashSet<string> for Names check. Because ResourceFileReader::GetResourceStream is called multiple times.
  For example. it is called about 443 times for samples/seed project.
  And Names contains 71/93 elements (For default/modern templates).

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bfefe72) 77.57% compared to head (50e9703) 77.58%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9090   +/-   ##
=======================================
  Coverage   77.57%   77.58%           
=======================================
  Files         592      592           
  Lines       24606    24608    +2     
=======================================
+ Hits        19088    19091    +3     
+ Misses       5518     5517    -1     

Files Changed	Coverage Δ
src/Docfx.Build.Engine/DocumentBuilder.cs	79.53% <100.00%> (-0.12%)	⬇️
...ine/ResourceFileReaders/CompositeResourceReader.cs	88.88% <100.00%> (ø)
...ine/ResourceFileReaders/LocalFileResourceReader.cs	81.81% <100.00%> (+1.81%)	⬆️

... and 2 files with indirect coverage changes

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[yufeih]

Aha, improper use of LINQ easily cause perf problems.