Improve logic of calculating a "fully qualified name"

[srawlins]

For improvements that I'm working on in another CL, it made sense to tidy up this code. The _buildFullyQualifiedName function was unnecessarily recursive and complex. And then the _fullyQualifiedNameWithoutLibrary function (a) has a complex name and (b) also worked further to do undo some work done in _buildFullyQualifiedName. This CL tidies these two "fields".

• Combine fullyQualifiedName and _buildFullyQualifiedName into one getter (removing one field).
• Combine fullyQualifiedNameWithoutLibrary and _fullyQualifiedNameWithoutLibrary into one late final field, remove recursion, and simplify name to qualifiedName. Also mark as @visibleForOverriding. It should be private but because of the mixin hierarchy, it is declared in one file, and defined in another.
• Improve documentation.

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[srawlins]

CC @dart-lang/analyzer-team