Started working on having the ability to have a different license for each version of a crate.

[baileyn]

This is some work towards fixing #736

[carols10cents]

Looks like rustfmt is still sad :( I'm not sure why it's different... I'm going to try running it on here.

[carols10cents]

This is going well!! I love the VersionBuilder in the tests ❤️

I tried out the code locally, and publishing versions locally was doing what I would expect as far as continuing to update the crate license, continuing to reject invalid licenses, and starting to keep track of the licenses in the versions table 💯

One small issue I found when running update-licenses however: the license column on the crate table can be null (that is, we haven't put a NOT NULL restriction on it in the database), so of course that means there are some crates with a null license in the production database (36 of them. and somehow one in my local database??) 😱😱😱

I'm going to file a separate issue for that, because I think we should be able to make it not null, but I'm not sure how it's happening, if it's something that's been fixed or not, so I think in this PR we should support a null license.

Currently, though, when update-licenses sees a null value, I get this error:

thread 'main' panicked at 'error retrieving column "license": Conversion(WasNull)', /Users/carolnichols/.cargo/registry/src/github.com-1ecc6299db9ec823/postgres-0.14.0/src/rows.rs:206
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::panicking::default_hook::{{closure}}
   2: std::panicking::default_hook
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic
   5: std::panicking::begin_panic_fmt
   6: postgres::rows::Row::get
   7: update_licenses::transfer
   8: update_licenses::main
   9: __rust_maybe_catch_panic
  10: std::rt::lang_start
  11: main

I think on line 30 of update-licenses.rs we can just change the type annotation from String to Option<String> and it should work? Do you mind trying that out and letting me know?

Since I'm not sure how crates.io is creating crates with null licenses, you can change a crate in your database to have a null license in psql by running:

UPDATE crates SET license = NULL where crates.name='whatever';

Thank you!!! Let me know if you have questions!!!

[carols10cents]

nice use of if let!

[carols10cents]

Looks great and the script works great with null licenses now too! ❤️