fix(load_plugins): ignore plugin whose name is started with "hexo-theme"

[stevenjoezhang]

What does it do?

When there are multiple hexo themes in package.json, the ones that are not activated will be loaded as plugins, which will cause errors

See also #4111

The new loading logic is to ignore all hexo-theme-* plugins, not only hexo-theme-${customThemeName}

However this goes back to the plugin name issue discussed in November 2019: #3890 (comment)

See also #4449

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.002%) to 98.306% when pulling 99d1c38 on theme into 35ca7bb on master.