Build local packages

[jyn514]

• Add a flag to make it possible
• Add documentation to the readme (since docker-compose makes it not very intuitive)

Note: this breaks backwards compatibility for build_package. This could be avoided by adding a private build_package_impl function but that seems ugly :/ I'm open to suggestions on how to make this backwards compatible.

[jyn514]

Motivation: someone asked in the docs-rs channel on discord whether they could see what docs.rs would do with a given package without publishing it on crates.io. As it currently stands, the answer was no. I would have found this useful in the past as well, so I added it.

[jyn514]

So after testing this locally I found that this crashes the web server once you try to actually load one of the local crates:

thread '<unnamed>' panicked at 'error retrieving column 4: Error(Conversion(WasNull))', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/postgres-0.15.2/src/rows.rs:201:31
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.34/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
   7: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:384
   8: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:339
   9: postgres::rows::Row::get
  10: cratesfyi::web::releases::get_releases
  11: <F as iron::middleware::Handler>::handle
  12: <alloc::boxed::Box<dyn iron::middleware::Handler> as iron::middleware::Handler>::handle
  13: router::router::Router::handle_method
  14: <router::router::Router as iron::middleware::Handler>::handle
  15: iron::middleware::Chain::continue_from_handler
  16: <iron::middleware::Chain as iron::middleware::Handler>::handle
  17: <alloc::boxed::Box<dyn iron::middleware::Handler> as iron::middleware::Handler>::handle
  18: <cratesfyi::web::CratesfyiHandler as iron::middleware::Handler>::handle
  19: <iron::iron::RawHandler<H> as hyper::server::Handler>::handle
  20: hyper::server::Worker<H>::handle_connection
  21: hyper::server::listener::spawn_with::{{closure}}

The root cause is that local packages were never released, so they don't have a release time. The fix is to make Release.release_time nullable.

[jyn514]

Actually a better idea is to keep the time non-nullable and set the release time for local packages to the build time

[jyn514]

This is ready for review.

[emilyalbini]

This looks great! Thanks!