Provide reason for unconverted bp2build modules

This also changes the expectation of ConvertWithBp2build. Each
implementation must either create one or more Bazel target modules, or
mark the module as unconvertible (with a specific reason).

Manually verified no runtime hit in AOSP
In AOSP, the metrics file size increases from 252K to 1.6M

This changes some effective module counts in bp2build metrics:
 - Removes "package" modules from the module count list in
metrics, as these will not be converted like regular modules.
 - Counts Handcrafted modules as being "unconverted", as bp2build is not
   responsible for them.

Bug: 285631638
Test: Verified generated BUILD.bazel files are bit-for-bit identical
with this change
Test: Manually verified one case of each implemented reasonType

Change-Id: I308dd451d8f28379b15671dae9f931bd0446f5c1