Add support for dependencies that aren't package names

[cottsay]

To date, all dependencies throughout the colcon codebase are always identified by package name, which aligns with the fact that all job identifiers are also package names. To support scenarios where there are jobs which aren't identified by package names, we'll also need to specify dependencies which aren't package names.

To eventually support this scenario, all primary shells (which implement command_prefix templates) should make a similar change to support the new attribute on the DependencyDescriptor class.

Note that at present, dependencies may also be bare strings, though DependencyDescriptor instances are preferred.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.00%. Comparing base (d03fa25) to head (89c0a3b).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
colcon_core/dependency_descriptor.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #724   +/-   ##
=======================================
  Coverage   86.99%   87.00%           
=======================================
  Files          69       69           
  Lines        4091     4100    +9     
  Branches      707      707           
=======================================
+ Hits         3559     3567    +8     
- Misses        421      422    +1     
  Partials      111      111           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Blast545]

Small clarification comments

[Blast545]

Thanks for the detailed answers to my comments, LGTM