compilepkg: skip duplicate cgo compile (#4645)

Coverage+cgo packages currently spend extra CPU in compilepkg because
nogo needs original cgo-generated Go sources, not the
coverage-instrumented files passed to the compiler. The old path called
cgo2 a second time to produce those sources. That second call also
recompiled every generated and user C-family source, linked the cgo
probe, and ran dynimport even though compilepkg discarded the object
outputs.

Split cgo source generation from the compile/link phase and add a
generate-only path for the nogo copy. Normal cgo compilation still uses
the full path, while the coverage+nogo pass writes generated Go sources
from a separate cgo_nogo work directory and reuses the real
_cgo_imports.go when available.