Find All References missing usages

[saul]

Find All References seems to be missing references to many symbols.

Note how in the first screenshot, Find All References is missing symbols that are known to the highlights service.

Repro steps

1. Select a symbol
2. Use Find All References

Expected behavior

All references to a symbol are shown.

Actual behavior

[vasily-kirichenko]

Fixed in #2139

However, Workspace does not aware of all projects in solution, see #1975, a project is added to Workspace when a file belonging to it is opened.

[cartermp]

Closing this, as #1975 does capture the issue of cross-project problems.