Tidy dependency checks cleanups + QoL

[clubby789]

• Refactors the list of workspaces into a documented struct
• Provide accurate line info in 'Go to ..... for the list' message
• Print crate name on dependency issue (i.e. dependency for rustc-main instead of dependency for .

[rustbot]

The list of allowed third-party dependencies may have been modified! You must ensure that any new dependencies have compatible licenses before merging.

cc @davidtwco, @wesleywiser

[rustbot]

r? @jieyouxu

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