refactor: ensure paths are safely passed around in ng-update

[devversion]

Schematics using the TypeScript compiler API have a significant issue
with paths. The problem is that the TypeScript compiler API intends to
rely fully on absolute paths, while ng-update and the devkit tree intend
to rely on project-scoped/relative paths.

This is problematic as it is straightforward to mix paths up, leading to
unexpected errors. For example, computing a relative path between a devkit
path and an absolute disk path results in a non-existent path.

To fix this ambiguity, we improve type safety of paths by using branded
primitives such as the @angular-devkit/core Path or an even more
strict type called WorkspacePath. A normal string is considered an
unresolved path and cannot be assigned to a WorkspacePath. Similarly a
WorkspacePath cannot be assigned to a string. This ensures that
resolved paths are not accidentally mixed with unresolved paths, and
that we can safely pass around unresolved and resolved paths without
loss of their semantics.

This fixes a case (and potentially more) where a devkit path and an absolute disk
path result in an incorrectly resolved resource. In CLI apps it's possible to reference
resources through absolute project-scoped paths. e.g. /src/app/app.component.html. This has been fixed, and tests have been added (this might be the fix for #19354)

Note: Long-term it would be ideal to implement a TypeScript compiler host that only deals with resolved workspace paths, but this is not possible yet as implementing TypeScript's readDirectory host method with a virtual tree is complex (and TS doesn't expose its helpers here).

[crisbeto]

LGTM. It seems like something we can easily forget, but I don't see what we can do about it, aside from some kind of linting.

[jelbourn]

LGTM

The difference between relative and absolute paths reminds me of https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function

Are there issues we should file with either the CLI or TypeScript to push the ecosystem in a more consistent direction?

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