build(bazel): Emit metadata.json under Blaze

[kyliau]

This commit modifies the compilation to emit metadata.json files when
compiled under Blaze. The default behavior of ngc is to emit metadata
only when the --flatModuleOutFile flag is specified, but this mode
is not used in Blaze.
Emitting metadata for individual Angular components is needed for
Angular Language Service to work with projects compiled with Blaze.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Metadata.json files not emitted when compiled under Blaze.

Issue Number: N/A

What is the new behavior?

Emit metadata.json for individual Angular components in Blaze.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[alexeagle]

Looks fine to me, would be nice to get Chuck to take a look too

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