External repositories: document legitimate path assumptions

Add a series of tests verifying the assumptions about execroot layout
a user or rule author may rely on. This serves a double purpose: one
the one hand, we verify that these legitimate assumptions stay stable,
on the other hand, we document what assumptions may be made with the
implicit understanding that anything not asserted may change in the
future without warning.

This patch brings the first set of legitimate assumptions, those about
path that may unconditionally be used.
- Each target may refer to true source files in its own repository
  via paths relative to its repository root.
- A library can be #inlcude'd via path relative to the root of the
  repository it resides in.

In a follow-up patch, the legitimate assumptions about the portability
of paths obtained from Skylark will be documented.

Change-Id: I6d91fa4e6e3e2e8040a778fe07425bdd8532b94b
PiperOrigin-RevId: 186272067

0 files changed, 0 insertions, 0 deletions