[WIP] feat: refactor fusion

[ClarkXia]

• build-plugin-fusion 仅维护 babel-plugin-import 和 externals 逻辑
• 新增 build-plugin-fusion-legacy 处理旧版主题方案及历史兼容逻辑

[codecov-commenter]

Codecov Report

Merging #4462 (65b3c3a) into release/2.0.0 (89f344a) will increase coverage by 3.69%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           release/2.0.0    #4462      +/-   ##
=================================================
+ Coverage          18.05%   21.75%   +3.69%     
=================================================
  Files                 43       42       -1     
  Lines               1030      855     -175     
  Branches             233      175      -58     
=================================================
  Hits                 186      186              
+ Misses               844      669     -175     

[ClarkXia]

后续增量模版不再推荐直接使用 fusion 插件，相关讨论 #4705 #4703