Correct null dereference exception on unusual import URIs

[jcollins-g]

Fixes #2426.

BREAKING CHANGE for PackageGraph.allLibraries (change of key type).

Import URIs that resolve via unusual paths, but otherwise map to the "same" library, result in a null pointer exception when traversing the import/export graph. Fix this by forcing resolution to LibraryElements generated for the normalized URIs.

[coveralls]

Coverage increased (+0.01%) to 91.411% when pulling 8dd24bb on npe-importexportlib into 847121d on master.

[CaptainIRS]

I think it would be useful to emit a warning like the no-defining-library-found warning when an unusual URI is encountered.

dartdoc/lib/src/warnings.dart

Lines 152 to 157 in 6d3c360

	PackageWarning.noDefiningLibraryFound: PackageWarningDefinition(
	PackageWarning.noDefiningLibraryFound,
	'no-defining-library-found',
	'The defining library for an element could not be found; the library may '
	'be imported or exported with a non-standard URI',
	defaultWarningMode: PackageWarningMode.error),

[srawlins]

Why declare this on line 891 and then initialize it on 892?

[jcollins-g]

No good reason, I just rewrote these sections of code too many times working on this. Fixed.

[jcollins-g]

I think it would be useful to emit a warning like the no-defining-library-found warning when an unusual URI is encountered.

dartdoc/lib/src/warnings.dart

Lines 152 to 157 in 6d3c360

	PackageWarning.noDefiningLibraryFound: PackageWarningDefinition(
	PackageWarning.noDefiningLibraryFound,
	'no-defining-library-found',
	'The defining library for an element could not be found; the library may '
	'be imported or exported with a non-standard URI',
	defaultWarningMode: PackageWarningMode.error),

I'm not entirely opposed to the idea, but I wonder if that might better belong in the linter. Filed as #2431.