keyword as submodule name in 0.12

[chikei]

minimal repro: https://github.com/chikei/mill-keyword-package

if I declare package using package build.`import`
mill complains:

1 tasks failed
generateScriptSources Package declaration "package build.`import`" in import/package.mill does not match folder structure. Expected: "package build.import"
Package declaration "package build.`import`" in import/package.mill does not match folder structure. Expected: "package build.import"

if declare using package build.import
mill complains differently:

1 tasks failed
generateScriptSources import/package.mill:1:14 expected end-of-input
package build.import
             ^
import/package.mill:1:14 expected end-of-input
package build.import

note that submodule like package build.`name-hyphen` generally fine