Fix the bug that host classes could be loaded from the plugin class loader.

[luanwenfei-venus]

【Fix issue】#1352

[Modification content] 1. Added the plug-in class loader to load from itself first. 2. Modified the cache field name for plug-in service class loading.

[Use case description] Not involved

[Self-test situation] 1. The local static check has been cleared; 2. The threshold use case passed

[Scope of influence] Not involved

[codecov]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (e103574) 42.83% compared to head (c9b536d) 42.76%.

Additional details and impacted files

@@             Coverage Diff              @@
##              1.2.x    #1356      +/-   ##
============================================
- Coverage     42.83%   42.76%   -0.07%     
+ Complexity     3301     3298       -3     
============================================
  Files           824      824              
  Lines         18311    18322      +11     
  Branches       2015     2017       +2     
============================================
- Hits           7844     7836       -8     
- Misses         9484     9504      +20     
+ Partials        983      982       -1     

Flag	Coverage Δ
unittests	42.76% <0.00%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...nt/core/plugin/classloader/ServiceClassLoader.java	0.00% <0.00%> (ø)
...ant/core/plugin/classloader/PluginClassLoader.java	0.00% <0.00%> (ø)

... and 20 files with indirect coverage changes

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