Remove ArgumentError() in parse_cache_header() when @depot cannot be resolved

[staticfloat]

In the original implementation of relocatable package cache files, parse_cache_header() was made responsible for resolving @depot/-relative paths, and it threw an ArgumentError() if it could not find one. However, this could happen for a few reasons:

1. The JULIA_DEPOT_PATH could be set incorrectly, so the appropriate source text files could not be found in an appropriate packages/ directory.

2. The package could have been upgraded, leaving a cache file for an older version on-disk, but no matching source files.

For a concrete example of (2), see the following bash script:

#!/bin/bash
set -euo pipefail

TEMP_PATH=$(mktemp -d)
JULIA=${JULIA:-julia}
echo JULIA: ${JULIA}
export JULIA_DEPOT_PATH="${TEMP_PATH}"
trap 'rm -rf ${TEMP_PATH}' EXIT

# Create a `Foo.jl` that has an `internal.jl` within it
FOO_DIR=${JULIA_DEPOT_PATH}/dev/Foo
mkdir -p ${FOO_DIR}/src
cat >${FOO_DIR}/Project.toml <<EOF
name = "Foo"
uuid = "00000000-0000-0000-0000-000000000001"
version = "1.0.0"
EOF
cat >${FOO_DIR}/src/Foo.jl <<EOF
module Foo

include("internal.jl")
end
EOF

cat >${FOO_DIR}/src/internal.jl <<EOF
# Nothing to see here, folks
EOF

${JULIA} -e "import Pkg; Pkg.develop(\"Foo\"); Pkg.precompile()"

# Change `Foo` to no longer depend on `internal.jl`; this should invalidate its cache files
cat >${FOO_DIR}/src/Foo.jl <<EOF
module Foo
end
EOF
rm -f ${FOO_DIR}/src/internal.jl

# This should print `SUCCESS!`, not `FAILURE!`
${JULIA} -e 'using Foo; println("SUCCESS!")' || echo "FAILURE!"

This pull request removes the ArgumentError(), as it seems reasonable to require parse_cache_header() to not throw errors in cases like these. We instead rely upon a higher level validation to reject a cachefile whose depot cannot be found, which should happen when stale_cachefile() later checks to ensure that all includes are found on-disk, (which will be false, as these paths start with @depot/, an unlikely path prefix to exist).

[staticfloat]

Added a test

[fatteneder]

This looks good.

And I think this solves the problem of silent PkgEval skips which I discovered yesterday in #51746 (comment).
[ But it does not address the real problem there which IIUC is the use of dep/build.jl in the pkgs. ]