analyzer: make fields final, in pubspec lint rule code

Work towards #50986

Various classes have mutable fields which should be final. Some of the
fixes require using factory constructors:

* _PSDependency, _PSEnvironment, _PSGitRepo, _PSHost, _Pubspec
* _PSNode: resourceProvider can be private and non-null
* Doc comments tidied here and there

Change-Id: Ib11a059e72670c80a4172e1c688493c1067328c8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429582
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: srawlins