[Enhanced parts] Deferred import in part file fully shadows one in a parent file

[sgrekhov]

Please see https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Parts-with-imports/scope_A06_t02.dart and its parts:
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Parts-with-imports/scope_A06_t02_part1.dart
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Parts-with-imports/scope_A06_t02_part2.dart

part1 imports one deferred library as l. part2 imports another deferred library as l. In part2 call of member that exists in the library imported in part1 but absent in the library imported in part2 leads to compile-time error. Is this an issue or expected behavior?

Note: the output below is obtainet with dart-lang/co19#3365 applied. The current output (with no changes from this PR) a slightly different.

$ python3 tools/test.py -n analyzer-asserts-linux co19/LanguageFeatures/Parts-with-imports/scope_A06_t02
No build targets found.
Test configuration:
    analyzer-asserts-linux(architecture: x64, compiler: dart2analyzer, mode: release, runtime: none, system: linux, enable-asserts, use-sdk)
Suites tested: co19

FAILED: dart2analyzer-none release_x64 co19/LanguageFeatures/Parts-with-imports/scope_A06_t02
Expected: Pass
Actual: CompileTimeError

--- Command "dart2analyzer" (took 04.000042s):
DART_CONFIGURATION=ReleaseX64 out/ReleaseX64/dart-sdk/bin/dart out/ReleaseX64/gen/dartanalyzer.dart.snapshot -Dtest_runner.configuration=analyzer-asserts-linux --enable-experiment=enhanced-parts --ignore-unrecognized-flags --packages=/home/sgrekhov/Google/dart-sdk/sdk/.dart_tool/package_config.json --format=json /home/sgrekhov/Google/dart-sdk/sdk/tests/co19/src/LanguageFeatures/Parts-with-imports/scope_A06_t02.dart

unexpected analysis errors in scope_A06_t02_part2.dart:
- Line 46, column 22: COMPILE_TIME_ERROR.UNDEFINED_PREFIXED_NAME
  The name 'counter' is being referenced through the prefix 'l', but it isn't defined in any of the libraries imported using that prefix.

--- Re-run this test:
python3 tools/test.py -n analyzer-asserts-linux co19/LanguageFeatures/Parts-with-imports/scope_A06_t02
[00:04 | 100% | +    0 | -    1]

=== 0 tests passed, 1 failed ===

[lrhn]

Expected behavior.

A deferred import never combines with any other import in any way. Its prefix name only contains to its own imported names, and it shadows import/prefix names in the parent file.
Any import or prefix declared in a part file will completely shadow the deferred prefix.

It works that way to ensure that prefix.loadLibrary() controls all identifiers accessible through that prefix.

(I created some tests too, which haven't landed yet: https://dart-review.googlesource.com/c/sdk/+/452101, I think this is tested there.)