Allow for disambiguation of identical file names in different directories when using `--file-mark`

**Describe the problem/challenge you have**
When using the `--file-mark` flag, only a filename is able to be specified, from one of the `path`s specified with the `-f, --file` flag. E.g., given the following directory structure:

```
a/
b/
some-file.yaml
dataDir/
some-file.yaml
```

with the following contents in `b/some-file.yaml`:
```
#@ load("@ytt:data", "data")
#@ load("@ytt:yaml", "yaml")
#@ load("@ytt:struct", "struct")
#@ vals = struct.encode(yaml.decode(data.read("some-file.yaml")))

data: #@ vals.foo.bar
```

and the following contents in `dataDir/some-file.yaml`:
```
foo:
bar: "baz"
```

Running `ytt -f a/b -f a/dataDir/ --file-mark some-file.yaml:type=data --output-files foo --debug` (note use of the `--debug` flag) produces the following output:
```
- (private false)
files:
- some-file.yaml
- some-file.yaml
libraries:

total: 409.132µs
```

with no files in directory `foo`, where the intended output would be a file `foo/some-file.yaml` with contents:
```
data: baz
```

If we rename `b/some-file.yaml` to `b/other-file.yaml`, and run the same command, the output is now:
```
- (private false)
files:
- other-file.yaml
- some-file.yaml
libraries:

## file other-file.yaml (opts yamlmeta.DocSetOpts{WithoutComments:false, Strict:false, AssociatedName:"other-file.yaml"})
## file other-file.yaml (opts yamlmeta.DocSetOpts{WithoutComments:false, Strict:false, AssociatedName:"other-file.yaml"})
## file other-file.yaml (opts yamlmeta.DocSetOpts{WithoutComments:false, Strict:false, AssociatedName:"other-file.yaml"})
### ast (templated)
????: docset (obj=0xc00022eb60)
1: doc (obj=0xc0001b6cd0)
1: map (obj=0xc0001b6c80)
6: key=data (obj=0xc0001829c0)
comment: 1: '@ load("@ytt:data", "data")'
comment: 2: '@ load("@ytt:yaml", "yaml")'
comment: 3: '@ load("@ytt:struct", "struct")'
comment: 4: '@ vals = struct.encode(yaml.decode(data.read("some-file.yaml")))'
comment: 6: '@ vals.foo.bar'
:
### template
src: tmpl: code: | srccode
????: 1: __ytt_tpl2_set_ctx_type("yaml") |
????: 2: __ytt_tpl2_start_ctx("yaml") |
????: 3: __ytt_tpl2_start_node(1) # *yamlmeta.DocumentSet |
1: 4: __ytt_tpl2_start_node(2) # *yamlmeta.Document | #@ load("@ytt:data", "data")
1: 5: __ytt_tpl2_start_node(3) # *yamlmeta.Map | #@ load("@ytt:data", "data")
1: 6: load("@ytt:data", "data") | #@ load("@ytt:data", "data")
2: 7: load("@ytt:yaml", "yaml") | #@ load("@ytt:yaml", "yaml")
3: 8: load("@ytt:struct", "struct") | #@ load("@ytt:struct", "struct")
4: 9: vals = struct.encode(yaml.decode(data.read("some-file.yaml"))) | #@ vals = struct.encode(yaml.decode(data.read("some-file.yaml")))
6: 10: __ytt_tpl2_start_node(4) # *yamlmeta.MapItem key=data | data: #@ vals.foo.bar
6: 11: __ytt_tpl2_set_node(4, (vals.foo.bar)) # *yamlmeta.MapItem key=data | data: #@ vals.foo.bar
????: 12: __ytt_tpl2_end_ctx(None) | ### other-file.yaml result
data: baz
creating: foo/other-file.yaml
total: 2.204487ms
```

which produces the expected output file `foo/other-file.yaml`.

**Describe the solution you'd like**
Allow `--file-mark` to accept absolute paths, or introduce an additional flag, something like `--file-mark-absolute` which allows for absolute paths to files to be marked.

**Anything else you would like to add:**
This issue is similar to the discussion at https://github.com/carvel-dev/ytt/discussions/326, which is, in my opinion, fundamentally the same issue, but the proposed approach is a bit more specific; to allow for OS paths to specify filenames. Note as well that the discussion does not appear to have ever been resolved.

---
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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start at the CLI handling for --file-mark and reproduce the a/b and a/dataDir example with --debug. Trace how marked files are selected when basenames collide, then define and test unambiguous path-based selection with the requested output behavior.

Written by the indexing model from the issue text.