This commit adds a check to `can_combine`, which is called by the
`SourceScanOptimizer` when determining whether it can combine two
branches into one.

The new check makes sure we only combine branches if:
- both branches don't share aliases
- if they do share aliases, the aliases refer to different things

This commit adds dataflow and SQL snapshot tests that verify we're not
generating duplicated aliases when combining branches.

I created 3 new dummy metrics for these tests: `derived_sharedalias_1a`,
`derived_sharedalias_1b` and `derived_sharedalias_2`. The `1a` and `1b`
variant both make `shared_alias` refer to `bookings`, while the `2`
variant makes it refer to `instant_bookings`. This way we can test the 2
cases:
- If the alias refers to the same thing (`1a` vs `1b`), we can merge
branches safely
- If the alias refers to different things (`1a` vs `2`), we cannot merge
branches safely

This commit adds a new integration test that uses
`derived_sharedalias_1a` and `derived_sharedalias_2`.

I manually optimized the SQL from the output of the optimized SQL
snapshot.