Fix import error caused by invalid file name

[phillebaba]

Turns out that Go has file naming conventions which are not enforced when working locally, but errors when importing packages. Prefixing all blob test files with sha256: would cause the import to error.

go: github.com/phillebaba/spegel-import imports
	github.com/spegel-org/spegel/pkg/oci: create zip: pkg/oci/testdata/blobs/sha256:0ad7c556c55464fa44d4c41e5236715e015b0266daced62140fb5c6b983c946b: malformed file path "pkg/oci/testdata/blobs/sha256:0ad7c556c55464fa44d4c41e5236715e015b0266daced62140fb5c6b983c946b": invalid char ':'
pkg/oci/testdata/blobs/sha256:3caa2469de2a23cbcc209dd0b9d01cd78ff9a0f88741655991d36baede5b0996: malformed file path "pkg/oci/testdata/blobs/sha256:3caa2469de2a23cbcc209dd0b9d01cd78ff9a0f88741655991d36baede5b0996": invalid char ':'
pkg/oci/testdata/blobs/sha256:44cb2cf712c060f69df7310e99339c1eb51a085446f1bb6d44469acff35b4355: malformed file path "pkg/oci/testdata/blobs/sha256:44cb2cf712c060f69df7310e99339c1eb51a085446f1bb6d44469acff35b4355": invalid char ':'
pkg/oci/testdata/blobs/sha256:68b8a989a3e08ddbdb3a0077d35c0d0e59c9ecf23d0634584def8bdbb7d6824f: malformed file path "pkg/oci/testdata/blobs/sha256:68b8a989a3e08ddbdb3a0077d35c0d0e59c9ecf23d0634584def8bdbb7d6824f": invalid char ':'
pkg/oci/testdata/blobs/sha256:9430beb291fa7b96997711fc486bc46133c719631aefdbeebe58dd3489217bfe: malformed file path "pkg/oci/testdata/blobs/sha256:9430beb291fa7b96997711fc486bc46133c719631aefdbeebe58dd3489217bfe": invalid char ':'
pkg/oci/testdata/blobs/sha256:9506c8e7a2d0a098d43cadfd7ecdc3c91697e8188d3a1245943b669f717747b4: malformed file path "pkg/oci/testdata/blobs/sha256:9506c8e7a2d0a098d43cadfd7ecdc3c91697e8188d3a1245943b669f717747b4": invalid char ':'
pkg/oci/testdata/blobs/sha256:addc990c58744bdf96364fe89bd4aab38b1e824d51c688edb36c75247cd45fa9: malformed file path "pkg/oci/testdata/blobs/sha256:addc990c58744bdf96364fe89bd4aab38b1e824d51c688edb36c75247cd45fa9": invalid char ':'
pkg/oci/testdata/blobs/sha256:aec8273a5e5aca369fcaa8cecef7bf6c7959d482f5c8cfa2236a6a16e46bbdcf: malformed file path "pkg/oci/testdata/blobs/sha256:aec8273a5e5aca369fcaa8cecef7bf6c7959d482f5c8cfa2236a6a16e46bbdcf": invalid char ':'
pkg/oci/testdata/blobs/sha256:d8df04365d06181f037251de953aca85cc16457581a8fc168f4957c978e1008b: malformed file path "pkg/oci/testdata/blobs/sha256:d8df04365d06181f037251de953aca85cc16457581a8fc168f4957c978e1008b": invalid char ':'
pkg/oci/testdata/blobs/sha256:dce623533c59af554b85f859e91fc1cbb7f574e873c82f36b9ea05a09feb0b53: malformed file path "pkg/oci/testdata/blobs/sha256:dce623533c59af554b85f859e91fc1cbb7f574e873c82f36b9ea05a09feb0b53": invalid char ':'

This change removes the prefix and adjusts the OCI tests to work with the new file paths.

[phillebaba]

I have verified that importing now works by importing this commit.