Add option to fail if a crate's license cannot be reasonably discovered

[mikayla-maki]

Hi y'all, thanks for creating this awesome project :)

As part of integrating this into our CI builds we wanted an option to fail CI if a crate's license could not be determined. I initially set out to implement #203 but quickly discovered that there are a lot of warnings generated for problems that get fixed in a later pass (e.g. failing to read a license identifier but successfully synthesizing one). I tried to put my check as late in the process as possible so that this flag only generates errors in truly unrecoverable situations. As such I think this PR captures the spirit of #203, if not the wording.

For testing I temporarily added this line as a dependency to cargo about:

procinfo = { git = "https://github.com/zed-industries/wezterm", rev = "5cd757e5f2eb039ed0c6bb6512223e69d5efc64d", default-features = false }

For whatever reason, this dependency's license cannot be detected by cargo about, probably because we're depending on a module in this non-library project rather than going through crates.io. Regardless, running cargo run generate --fail_on_missing_license about.hbs should now generate an error like so:

2023-01-23 19:58:56.211281 +00:00:00 [WARN] crate 'procinfo 0.1.0' doesn't have a license field
error: unable to synthesize license expression for 'procinfo 0.1.0': no `license` specified, and no license files were found

2023-01-23 19:59:01.615755 +00:00:00 [ERROR] encountered 1 errors resolving licenses, unable to generate output

Adding the following field to about.toml should fix said error:

[procinfo.clarify]
license = "MIT"
[[procinfo.clarify.git]]
path = 'LICENSE.md'
checksum = '37db33bbbd7348969eda397b89a16f252d56c1ca7481b6ccaf56ccdcbab5dcca'

Let me know if there's anything I can fix or reformat :)

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Adds a fail_on_missing_license flag to the CLI.

Related Issues

#203

[Jake-Shadle]

Thanks!