fix($core): cannot retrieve the correct theme name when them path is a local absolute path linked to a javascript file.

We shouldn't let the core render the `.content` class, so once we use custom layout in the default theme, the styles of default theme will pollute the styles of custom layout.

This reverts commit f04adbf1.

This reverts commit 760f90b9.

This reverts commit cab5b831.

fix($core): failed to load theme when using "layouts/Layout.vue" as entry file (close: #1563) (#1564)

ref: https://github.com/facebook/jest/pull/7687

fix($core): use directory name to compute slug if filename is readme or index (close: #1443) (#1535)

BTW, A theme cannot require the theme's styles at the layout but need use `Index.styl`  instead.

Also renaming __VUEPRESS_ROUTE_BASE__ to __VUEPRESS_ROUTER_BASE__