intellij中文件夹种类以及包名突然报错或者el、jstl等标签突然失效的修复

查阅 IntelliJ IDEA 2018.2 Help，内容如下（英语简单，就不用翻译了）：

Folders within content roots can be assigned to several categories.

Sources Root
Production code that should be compiled.
Generated Sources Root
If a folder is marked as generated, the IDE will think that files in this folder are generated automatically rather than written manually, and can be regenerated.
Test Sources Root
These folders allow you to keep code related to testing separately from production code. Compilation results for sources and test sources, normally, are placed into different folders.
Generated Test Sources Root
If a folder is marked as generated, the IDE will think that files in this folder are generated automatically rather than written manually, and can be regenerated.
Resources Root (available only in Java modules)
Resource files used in your application (images, configuration XML and properties files, etc.). During the build process, resource files are copied to the output folder as is.
Load Path Root (available only when the Ruby plugin is enabled)
The load path is the path where the require and load statements look for files.
Test Resources Root (available only in Java modules)
These folders are for resource files associated with your test sources.
Excluded
Files in excluded folders are ignored by code completion, navigation and inspection. That is why, when you exclude a folder that you don't need at the moment, you can increase the IDE performance.

Normally, compilation output folders are marked as excluded.

Apart from excluding the entire folders, you can also exclude specific files.