Be more explicit about what requires a package to be recompiled

[kit-ty-kate]

Request from @jonahbeckford (please feel free to correct me if i’m wrong)

Sometimes opam has to compile package. e.g.:

[build]   - recompile dkml-base-compiler                   4.14.0~v2.1.0 (pinned)         [upstream or system changes]

I feel like it would be nice to at least say what kind of change this is.

For example:

• a change in the git upstream for pinned package could be shown as git source changed: <old hash> to <new hash>
• a change in the opam file (e.g. changes in build, etc...) could be show as some fields changed in the opam file: build

[jonahbeckford]

Thx!