Refactor usage of llb.Local name and key hint

[hinshun]

• Removes solver.LocalPathDescriptionKey, it was used only to see what local's digest actually pointed to in the solver tree. Since the path is now part of the local name, this isn't necessary anymore. (https://github.com/openllb/hlb/pull/97#issuecomment-614268142)
• Use localPath as name for llb.Local, but under the hood use abs path for sync dir
• Use our deterministic ID in llb.SharedKeyHint instead, not visible to end user but gives us the same benefits
• Include filename in localPath when specifying a file directory with local "filename". Doesn't have any functional difference but makes the output better

Before

~/tmp
❯ cat foo.hlb
fs default() {
    local "foo.hlb"
}

~/tmp
❯ hlb run foo.hlb
[+] Building 0.0s (1/1) FINISHED
 => local://sha256:408dc57624f1359d2eaa3b6ef97dcff3b16d16125db02684613a33a8aec2bec0 (foo.hlb)
 => => transferring sha256:408dc57624f1359d2eaa3b6ef97dcff3b16d16125db02684613a33a8aec2bec0: 28B

After

~/tmp
❯ cat foo.hlb
fs default() {
    local "foo.hlb"
}

~/tmp
❯ hlb run foo.hlb
[+] Building 0.0s (1/1) FINISHED
 => local://foo.hlb (foo.hlb)
 => => transferring foo.hlb: 28B