Move the `CheckMountpoints()` implementation to `blueprint` package,
since it does not operate on any data structures from the `disk`.
Move the default mountpoint allow list policy definition to the
`pathpolicy` package.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
Move the `FilesystemCustomization` structure and its custom
unmarshallers to a dedicated file. This makes `customizations.go` easier
to read.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>