ccpp_prebuild.py: metadata2html.py (mkdoc.py) broken

[climbfuji]

Description

A PR last year that added an out-of-source build to the CCPP framework and removed/rearranged old files. During these steps, the usually untested metadata2html.py (which uses mkdoc.py) were broken. These are only used for updating scientific documentation, hence whenever there is a CCPP release.

Steps to Reproduce

Run metadata2html.py as described in the CCPP technical documentation, either in source or out of source.

Expected behavior

metadata2html.py runs successfully for both in source and out of source builds.