[Bug]: Temporary VRT fails for unzipped SAFE files due to `relativeToVRT=1`

Checked for duplicates

Yes - I've already checked

Describe the bug

When I ran COMPASS with unzipped files, I was getting read errors.
I checked the _temp.vrt file made for each burst:

$ gdalinfo unzipped/scratch/t087_185679_iw1/20180511/t087_185679_iw1_20180511_VV_temp.vrt

Driver: VRT/Virtual Raster
Files: unzipped/scratch/t087_185679_iw1/20180511/t087_185679_iw1_20180511_VV_temp.vrt
Size is 20492, 1490
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0, 1490.0)
Upper Right (20492.0,    0.0)
Lower Right (20492.0, 1490.0)
Center      (10246.0,  745.0)
Band 1 Block=128x128 Type=CFloat32, ColorInterp=Undefined
  NoData Value=0
ERROR 4: unzipped/scratch/t087_185679_iw1/20180511/S1A_IW_SLC__1SDV_20180511T161552_20180511T161622_021859_025BF8_1292.SAFE/measurement/s1a-iw1-slc-vv-20180511t161552-20180511t161620-021859-025bf8-004.tiff: No such file or directory

What did you expect?

I expected the VRT to correctly point to the .tiff file

Reproducible steps

The reason for the failure is here: https://github.com/opera-adt/s1-reader/blob/428328de5011968da91174cdf14db6eb3e8264c7/src/s1reader/s1_burst_slc.py#L360

We are using relativeToVRT=1, which means we're saying that the location of the tiff_path will be relative to the location of the VRT.
This is probably only going to be true when we do slc_to_vrt_file('.'), since we're passing some other path to the tiff_path.

The reason this is currently working for the zipped file testing is gdal ignores your indication of relativeToVrt when your path starts with a / and treats it as absolute

Note that "relativeToVRT=0" doesn't mean it has to be a full path. If it's not, then the path is treated relative to whatever directory you are in.

For example:

$ tree
.
├── data
│   └── swath.tiff
├── folder-with-vrt
│   ├── test_absolute.tiff
│   └── test_relative.tiff

 cat folder-with-vrt/test_relative.tiff
<VRTDataset rasterXSize="20492" rasterYSize="1490">
    <VRTRasterBand dataType="CFloat32" band="1">
        <NoDataValue>0.0</NoDataValue>
        <SimpleSource>
            <SourceFilename relativeToVRT="1">data/swath.tiff</SourceFilename>
            <SourceBand>1</SourceBand>
            <SourceProperties RasterXSize="20492" RasterYSize="14900" DataType="CInt16"/>
            <SrcRect xOff="15" yOff="8957" xSize="20459" ySize="1456"/>
            <DstRect xOff="15" yOff="17" xSize="20459" ySize="1456"/>
        </SimpleSource>
    </VRTRasterBand>
</VRTDataset>

$ gdalinfo folder-with-vrt/test_relative.tiff...

ERROR 4: folder-with-vrt/data/swath.tiff: No such file or directory

$ diff folder-with-vrt/test_absolute.tiff folder-with-vrt/test_relative.tiff
5c5
<             <SourceFilename relativeToVRT="0">data/swath.tiff</SourceFilename>
---
>             <SourceFilename relativeToVRT="1">data/swath.tiff</SourceFilename>

$ gdalinfo folder-with-vrt/test_absolute.tiff

(it works)

Start in src/s1reader/s1_burst_slc.py around line 360 and inspect how slc_to_vrt_file() writes the TIFF path and relativeToVRT setting. Reproduce the failure with an unzipped SAFE and inspect the generated _temp.vrt using gdalinfo. Done means the VRT resolves the TIFF correctly and the read errors no longer occur.