Adding a mount should keep the default mounts

feature
1

The module mount doc says:

When you add a mount, the default mount for the concerned target root is ignored: be sure to explicitly add it.

If I want to mount

[[module.mounts]]
source = "node_modules/bootstrap-icons/font/fonts"
target = "static/paige/bootstrap-icons/fonts"

then I also have to mount

[[module.mounts]]
source = "static"
target = "static"

[[module.mounts]]
source = "layouts"
target = "layouts"

[[module.mounts]]
source = "assets"
target = "assets"

just to avoid a build error, and I would guess that I should just do this for all mount directories listed in the doc anyway, just in case.

This is tedious and error-prone.

Could it work to instead have the default mounts stay in place unless they are explicitly overridden?

For example, if I want to override layouts, I can explicitly do

[[module.mounts]]
source = "my-layouts"
target = "layouts"

Otherwise, the default is

[[module.mounts]]
source = "layouts"
target = "layouts"

and I can mount things in /layouts easily:

[[module.mounts]]
source = "mything/foo"
target = "layouts/foo"

If someone explicitly sets the default, they get the current behavior. I don’t see a back compat issue with this change.