Add `Base.isrelocatable(pkg)` (#53906)
This PR adds a utility function `isrelocatable(pkg)` that can be used to
check if `pkg` is already precompiled and if the associated cachefile is
relocatable.
The reason to implicitly perform the `isprecompiled` check is that the
exact same computation needs to be done to find the right `.ji`.
A `pkg` is said to be relocatable if
1. all `include()` paths are relocatable (they start with `@depot`),
2. all `include_dependency()` paths are relocatable (they start with
`@depot` and `track_content=true` was used to include them).
