Make the bzip2 compress plugin a proper module

Our current practice of dlopening foreign shared libraries is problematic
for several reasons:

• not portable: modules and shared libraries can be different object types
• dependency information is invisible (our canaries mostly solve this)
• hardwiring SONAMES breaks on transitions (KNET_PKG_SONAME solves this)
• symbol versioning information is lost (theoretically solvable)

The preferred way out is generating dynamically loaded private modules
from the main source, which then rely on the dynamic linker to load the
external symbols as usual.