fix Issue 15900 - public imports not accessible using FQN

[MartinNowak]

• public imports in imported modules were not accessbile using their
  FQN, this was an oversight when adding the package tree masking to fix
  Bugzilla 313 (see fix Issue 313 - Fully qualified names bypass private imports #5426)
• fixed by recursively checking imported scopes for accessible packages
• Uses the same reduced visibility distinction (only private vs. rest) as the
  unqualified symbol search, b/c extending importedScopes to track rich
  visibility (e.g. package(a.b)) was out of scope for a regression fix.
  This should be implemented when combining the search/import w/
  the symbol visibility mechanism.
• also fixes Issue 16316 - FQN of imports in mixin template not accessible
  by searching the importedScopes for accessible packages

[dlang-bot]

Fix	Bugzilla	Description
✓	15900	[REG 2.071](Import deprecation) Public import ignored when using fully qualified name
✓	16316	[REG 2.071](Import deprecation) fully qualified name of imports in mixin template not accessible

[MartinNowak]

Need to fix Travis-CI testing of stable branch (it's checking out druntime/phobos) master atm., please ignore the failures, but only this time ;).

[MartinNowak]

Finally fixed all the testing issues with stable, both for Travis-CI and the auto-tester.

[wilzbach]

Finally fixed all the testing issues with stable, both for Travis-CI and the auto-tester.

FYI The windows machines still seem to fail :/

[WalterBright]

Auto-merge toggled on

[MartinNowak]

FYI The windows machines still seem to fail :/

Got fixed here, had to handle public import cycles.