fix(pkg_install): resolve files against own repository (#1016)

tl;dr: this change builds on #984 which introduced the `locate()` helper
and the `runfiles` infrastructure. That PR fixed Windows compatibility
by enabling runfiles-less operation; this change extends it to support
cross-repository files.

When a `pkg_install` rule in an external repository references files
from another repository, the installer would fail at runtime by
attempting to resolve all files relative to its repository rather than
each file's own repository.

Example failure (`extrepo`: external repository installing file from
main repository):
```
bazel run @extrepo//:install -- --destdir=/tmp/test
[...]
FileNotFoundError: [Errno 2] No such file or directory:
  '.../runfiles/+_repo_rules+extrepo/some-path/extrepo/file-in-main-repo'
```

The file should be resolved against the main repository (`_main`) but
is incorrectly looked up from the installer's repository
(`+_repo_rules+extrepo`).

The solution proposed here consists in extending the manifest to track
each file's own repository and use this information during installation
to resolve files against the correct repository.

It adds a `repository` field to manifest entries using:
- `Label.repo_name`
  ([canonical](https://bazel.build/rules/lib/builtins/Label#repo_name))
  when explicitly valued for the source file,
- otherwise `ctx.workspace_name` when no
  [owner](https://bazel.build/rules/lib/builtins/File#owner)
  or `Label.repo_name` is
  [empty](https://rules-python.readthedocs.io/en/latest/api/py/runfiles/runfiles.runfiles.html#runfiles.runfiles.Runfiles.CurrentRepository),
  denoting the main repository (`_main`, as before).

Testing:
- `//tests/install:install_test` includes new `CrossRepoInstallTest`,
- `bazel run @extrepo//:install -- --destdir=/tmp/test` case works,
- verified the above on Ubuntu Linux and Windows as well.