Converter should only compute default command once

Currently, if no command is set, the converter will compute the default command once per assembly. This is unnecessary work. Instead, the converter should update the build config object with the resolved value.

We may want to think about inverting the control and having the converter export the getDefautCommand function so that it can be computed in the assembleContent function, ahead of calling the convert function.

(If the converter still decided to alter the command, or if it doesn't export getDefautCommand, it should then update the property on the build config object as originally proposed.)