Exclude built plugins/themes from build process.

[ampersarnie]

When including a plugin or theme from a release that has build assets and the src directory still intact, the build tools will still attempt to compile and add those entrypoints to its process and list of projects to be compiled. These types of projects are not something we should be targeting for build when running through a compile process and need to be excluded unless explicitly stated.

Originally flagged as an issue here: https://github.com/bigbite/block-footnotes/issues/36

[g-elwell]

@ampersarnie there was an issue with our CI that meant files listed in deployignore weren't being removed when creating a built branch for a project.

Since this is now fixed, this issue may be dependent on whether you feel we should be excluding src directories from builds or not.